Projection screen and projection system containing same

ABSTRACT

A projection screen capable of sharply displaying an image even under bright environmental light with high image visibility. A projection screen has a polarized-light selective reflection layer with a cholesteric liquid crystalline structure, capable of selectively reflecting a specific polarized light component, and a substrate for supporting the reflection layer. Of the light entering the reflection layer from the viewer&#39;s side, the right-handed light in the selective reflection wave range is reflected from the reflection layer as reflected light. As a result of structural non-uniformity in the cholesteric liquid crystalline structure, the light that is selectively reflected (reflected light) is diffused.

This is a Continuation of application Ser. No. 10/864,876 filed Jun. 10,2004. The entire disclosure of the prior application is herebyincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a projection system in which imaginglight emitted from a projector is projected on a projection screen todisplay thereon an image. More particularly, the present inventionrelates to a projection screen capable of sharply displaying an imageand of providing high visibility, and to a projection system containingsuch a projection screen.

2. Description of Related Art

A conventional projection system usually operates as follows: imaginglight emitted from a projector is projected on a projection screen, andviewers observe the light reflected from the projection screen as animage.

Typical examples of projection screens for use in such conventionalprojection systems include white-colored paper or cloth materials, andplastic films coated with inks that scatter white light. High-qualityprojection screens that comprise scattering layers containing beads,pearlescent pigments, or the like, capable of controlling the scatteringof imaging light, also are now commercially available.

Projectors have become smaller in size and moderate in price in recentyears, so that not only demand for projectors for commercial use butalso demand for household projectors such as projectors for familytheaters is growing, and an increasing number of families are nowenjoying projection systems. Household projection systems are oftenplaced in living rooms or the like, which are usually so designed thatenvironmental light such as sunlight and light from lighting fixtures isabundant. Therefore, projection screens for use in household projectionsystems are expected to show good image display performance even underbright environmental light.

However, the above-described conventional projection screens cannot showgood image display performance under bright environmental light becausethe screens reflect not only imaging light but also environmental lightsuch as sunlight and light from lighting fixtures.

In such a conventional projection system, differences in the intensityof light (imaging light) projected on a projection screen from aprojector cause light and shade to form an image. For example, in thecase where a white image on a black background is projected, theprojected-light-striking part of the projection screen becomes white andthe other part becomes black; thus, differences in brightness betweenwhen and black cause light and shade to form the desired image. In thiscase, in order to attain excellent image display, it is necessary tomake-the contrast between the white- and black-indication parts greaterby making the white-indication part lighter and the black-indicationpart darker.

However, since the above-described conventional projection screenreflects both imaging light and environmental light such as sunlight andlight from light fixtures without distinction, both the white- andblack-indication parts get lighter, and differences in brightnessbetween white and black are decreased. For this reason, the conventionalprojection screen cannot satisfactorily provide good image displayunless the influence of environmental light such as sunlight and lightfrom lighting fixtures on the projection screen is suppressed by using ameans for shading a room, or by placing the projection screen in a darkenvironment.

Under these circumstances, studies have been made on projection screenscapable of showing good image display performance even under brightenvironmental light. There have so far been proposed projection screensutilizing holograms, polarized-light-separating layers, or the like (seeJapanese Laid-Open Patent Publication No. 107660/1993 (JP '660) andJapanese Laid-Open Patent Publication No. 540445/2002 (JP '445)).

Of these conventional projection screens, those using holograms have theadvantage that the white-indication part can be made lighter if thescattering of light is properly controlled, so that the screens can showrelatively good image display performance even under bright environmentlight. However, holograms have wavelength selectivity but nopolarization selectivity, meaning that the projection screens usingholograms can display images only with limited sharpness. Moreover, itis difficult to produce large-sized projection screens by utilizingholograms due to production problems.

On the other hand, using the above-described conventional projectionscreens with polarized-light-separating layers, it is possible to makethe white-indication part lighter and the black-indication part darker.Therefore, these projection screens can sharply display images evenunder bright environmental light as compared with the projection screensusing holograms.

Specifically, for example, JP '660 describes a projection screen havinga cholesteric liquid crystal that reflects red, green and blue light(right- or left-handed circularly polarized light) contained in imaginglight. This projection screen is made not to reflect nearly half theenvironmental light incident on the screen by making use of thecircularly-polarized-light-separating property of the cholesteric liquidcrystal.

However, in the projection screen described in JP '660, since thecholesteric liquid crystal is in the state of planar orientation,specular reflection occurs when the projection screen reflects light,and the reflected light cannot be well recognized as an image. Namely,to recognize the reflected light as an image, it is necessary that thereflected light be scattered. However, JP '660 is silent on this point.

On the other hand, JP '445 describes a projection screen using, as areflective polarization element, a multi-layered reflective polarizer orthe like, having diffusing power. This projection screen does notreflect part of the environmental light incident on the screen becauseof the polarized-light-separating property of the multi-layeredreflective polarizer, and diffuse-reflects the rest of the incidentlight due to interfacial reflection that occurs in the multi-layeredreflective polarizer composed of materials having different refractiveindices, or by means of a diffusing element provided separately from themulti-layered reflective polarizer. JP '445 also describes a projectionscreen using in combination a cholesteric reflective polarizer as areflective polarization element and a diffusing element. This projectionscreen does not reflect part of the environmental light incident on thescreen because of the polarized-light-separating property of thecholesteric reflective polarizer, and diffuse-reflects the rest of theincident light by means of the diffusing element provided separatelyfrom the cholesteric reflective polarizer.

Of the projection screens described in JP '445, the former one mustcontain a multi-layered reflective polarizer or the like that is alinear polarization element (“DBEF” manufactured by 3M Corporation,etc.). When this projection screen is incorporated into a projectionsystem or the like, it is necessary to make the plane of polarization ofthe linear polarization element agree with the plane of polarization ofa projector that emits linearly polarized light, such as a liquidcrystal projector. If these planes of polarization do not agree witheach other, excellent image display cannot be attained.

Further, of the projection screens described in JP '445 the latter onecontains, as the reflective polarization element, a circularpolarization element such as a cholesteric reflective polarizer.However, since the diffusing element for scattering the reflected lightis provided on the viewer's side of the reflective polarization element,the polarized-light-separating property of the reflective polarizationelement is impaired, and image visibility cannot be fully improved.

Namely, since the diffusing element is provided on the viewer's side ofthe reflective polarization element, light passes through the diffusingelement before entering the reflective polarization element, and itsstate of polarization is disturbed, which is called “depolarization.”Light that passes through the diffusing element includes two types oflight, that is, environmental light (sunlight, etc.) and imaging light.If the state of polarization of environmental light is disturbed by thediffusing element, the light that the reflective polarization elementinherently transmits is, because of depolarization, converted into acomponent that the reflective polarization element reflects, and thiscomponent is reflected from the reflective polarization element asunnecessary light. On the other hand, if the state of polarization ofimaging light is disturbed by the diffusing element, the light that thereflective polarization element inherently transmits is, because ofdepolarization, converted into a component that the reflectivepolarization element does not reflect, and this component passes throughthe reflective polarization element. Because of these two phenomena, theoriginal polarized-light-separating property is impaired, and imagevisibility cannot be fully improved.

Moreover, in the projection screens described in JP '660 and JP '445, itis necessary to provide anti-glaring layers in order to prevent theprojection screens from glaring. The polarized-light-separating propertyis impaired also by such anti-glaring layers.

In sum, the above-described conventional projection screens, includingthose ones using holograms and those ones described in JP '660 and JP'445, using polarized-light-separating layers, can display images onlywith limited sharpness under bright environmental light. Therefore, ithas so far been impossible to fully improve image visibility.

SUMMARY OF THE INVENTION

The present invention has been accomplished under these above-mentionedcircumstances. An object of the present invention is therefore toprovide a projection screen capable of sharply displaying an image evenunder bright environmental light and of providing intensified brightnessand high visibility, and a projection system containing such aprojection screen.

(Means for Fulfilling the Object)

A first aspect of the present invention is a projection screen thatdisplays an image by reflecting imaging light projected, containing apolarized-light selective reflection layer having a cholesteric liquidcrystalline structure and adapted selectively to reflect a specificpolarized light component, wherein the polarized-light selectivereflection layer selectively reflects the light component whilediffusing the light component as a result of structural non-uniformityin the cholesteric liquid crystalline structure.

In the above-described first aspect of the present invention, it ispreferable that the cholesteric liquid crystalline structure of thepolarized-light selective reflection layer includes a plurality ofhelical-structure parts that have different directions of helical axes.

Further, in the above-described first aspect of the present invention,it is preferable that the cholesteric liquid crystalline structure ofthe polarized-light selective reflection layer includes alayered-structure area in which planes of nematic layers are layered andedge-shaped-structure parts that are formed in the layered-structurearea by a partial edge dislocation of the planes of nematic layers, andthat the plurality of helical-structure parts that have differentdirections of helical axes are obtained due to directions of helicalaxes of the cholesteric liquid crystalline structure being changed inthe edge-shaped-structure parts and their vicinity.

Furthermore, in the above-described first aspect of the presentinvention, it is preferable that the plurality of helical-structureparts in the cholesteric liquid crystalline structure of thepolarized-light selective reflection layer contain, in one cross sectiontaken in a direction of a normal, that is, in a direction of a thicknessof the polarized-light selective reflection layer, thosehelical-structure parts in which the helical axes thereof are tiltedclockwise relative to the normal and those helical-structure parts inwhich the helical axes thereof are tilted counterclockwise relative tothe normal. In this case, in some of the helical-structure parts, thedirections of the helical axes thereof may be the same as the directionof the normal.

Furthermore, in the above-described first aspect of the presentinvention, it is preferable that the polarized-light selectivereflection layer selectively reflects light in a specific wave rangethat covers only a part of the visible region. More specifically, it ispreferable that the polarized-light selective reflection layer has, forlight in a wave range that covers only a part of the visible region,reflectivity not less than half the maximum reflectivity of this layer.Moreover, assuming that light enters the polarized-light selectivereflection layer vertically to it, it is preferable that thepolarized-light selective reflection layer selectively reflects light inwave ranges whose centers are between 430 nm and 460 nm, between 540 nmand 570 nm, and between 580 nm and 620 nm.

Furthermore, in the above-described first aspect of the presentinvention, it is preferable that the polarized-light selectivereflection layer contains at least two partial selective reflectionlayers laminated to each other, each of the partial selective reflectionlayers having a cholesteric liquid crystalline structure adaptedselectively to reflect a specific polarized light component and todiffuse the selectively reflected light as a result of structuralnon-uniformity in the cholesteric liquid crystalline structure, and theliquid crystalline structures of the partial selective reflection layersare different in helical pitch.

Furthermore, in the above-described first aspect of the presentinvention, it is preferable that the projection screen further includesa substrate that supports the polarized-light selective reflectionlayer. It is herein preferable that the substrate has a light-absorbinglayer adapted to absorb light in the visible region.

Furthermore, in the above-described first aspect of the presentinvention, it is preferable that the projection screen further containsan intermediate layer between the polarized-light selective reflectionlayer and the substrate, whereby liquid crystalline molecules in thecholesteric liquid crystalline structure of the polarized-lightselective reflection layer, present in a vicinity of a surface of theintermediate layer, are aligned so that directors of the liquidcrystalline molecules point in a plurality of directions. It is hereinpreferable that a intermediate layer be an adhesion-improving layer forimproving adhesion between the polarized-light selective reflectionlayer and the substrate. It is also preferable that the intermediatelayer has a light-absorbing layer adapted to absorb light in the visibleregion.

Furthermore, in the above-described first aspect of the presentinvention, it is preferable that the projection screen further includes,on a side of the substrate opposite to a side on which thepolarized-light selective reflection layer is provided, a pressuresensitive adhesive layer so that the substrate on which thepolarized-light selective reflection layer is provided can be affixed toan external member.

Furthermore, in the above-described first aspect of the presentinvention, it is preferable that the projection screen further contains,on a side of the substrate opposite to a side on which thepolarized-light selective reflection layer is provided, alight-reflecting layer for reflecting light that is incident on thesubstrate.

Furthermore, in the above-described first aspect of the presentinvention, it is preferable that the projection screen further has, onan outermost, viewer's side surface of the polarized-light selectivereflection layer, a hard coat layer for preventing the surface of theprojection screen from being scratched. It is herein preferable that thehard coat layer has a surface hardness of 2H or more when expressed bythe pencil hardness according to JIS K5400.

Furthermore, in the above-described first aspect of the presentinvention, it is preferable that the projection screen further contains,on a viewer's side of the polarized-light selective reflection layer, ananti-glaring layer for preventing the projection screen from glaring.

Furthermore, in the above-described first aspect of the presentinvention, it is preferable that the projection screen further has, on aviewer's side of the polarized-light selective reflection layer, ananti-reflection layer for preventing the projection screen fromreflecting extraneous light.

Furthermore, in the above-described first aspect of the presentinvention, it is preferable that the projection screen further includes,on a viewer's side of the polarized-light selective reflection layer, anultraviolet-absorbing layer adapted to absorb ultraviolet light incidenton the projection screen.

Furthermore, in the above-described first aspect of the presentinvention, it is preferable that the projection screen further includes,on at least one of viewer's side and backside surfaces of thepolarized-light selective reflection layer, an antistatic layer forpreventing the projection screen from being electrostatically charged.It is herein preferable that the antistatic layer has a surfaceresistivity of 1×10¹¹ Ω/□ or less.

Furthermore, in the above-described first aspect of the presentinvention, it is preferable that the polarized-light selectivereflection layer be made from a polymerizable liquid crystallinematerial.

A second aspect of the present invention is a projection systemcontaining a projection screen according to the above-described firstaspect of the present invention; and a projector that projects imaginglight on the projection screen.

In the above-described second aspect of the present invention, it ispreferable that the projection screen selectively reflects only light ina wave range that is identical with a wave range in which a imaginglight projected from the projector falls.

Further, in the above-described second aspect of the present invention,it is preferable that the imaging light to be projected on theprojection screen from the projector contains mainly a polarized lightcomponent that is identical to a polarized light component that theprojection screen selectively reflects.

Furthermore, in the above-described second aspect of the presentinvention, it is preferable that the projection system further includesan illuminant for illuminating a space in which the projection screen isplaced, the illuminant being so positioned that light emitted from theilluminant directly illuminates the projection screen, wherein lightemitted from the illuminant toward the projection screen contains mainlya polarized light component that is different from a polarized lightcomponent that the projection screen selectively reflects.

Furthermore, in the above-described second aspect of the presentinvention, it is preferable that the projection system further containsan illuminant for illuminating a space in which the projection screen isplaced, the illuminant being so positioned that light emitted from theilluminant indirectly illuminates the projection screen via a reflector,wherein the light emitted from the illuminant toward the reflectorcontains mainly a polarized light component that is identical to apolarized light component that the projection screen selectivelyreflects.

(Principle and Actions of the Present Invention)

According to the present invention, the projection screen includes apolarized-light selective reflection layer having a cholesteric liquidcrystalline structure and adapted selectively to reflect a specificpolarized light component, and the light component that is selectivelyreflected is diffused as a result of structural non-uniformity in thecholesteric liquid crystalline structure.

The polarized-light selective reflection layer selectively reflects onlya specific polarized light component (e.g., right-handed circularlypolarized light) because of the polarized-light-separating property ofthe cholesteric liquid crystalline structure, so that this layer can bemade to reflect only approximately 50% of the unpolarized environmentallight such as sunlight and light from light fixtures that are incidenton this layer. For this reason, while maintaining the brightness of thelight-indication part such as a white-indication part, it is possible tolower the brightness of the dark-indication part such as ablack-indication part to nearly half, thereby obtaining a nearlytwice-enhanced image contrast. In this case, if the imaging light to beprojected is made to contain mainly a polarized light component that isidentical with the polarized light component that the polarized-lightselective reflection layer selectively reflects (e.g., right-handedcircularly polarized light), the polarized-light selective reflectionlayer can reflect nearly 100% of the imaging light projected on thislayer, that is, this layer can efficiently reflect the imaging light.

Further, in the polarized-light selective reflection layer, thecholesteric liquid crystalline structure is structurally non-uniform,and the helical-structure parts contained in the cholesteric liquidcrystalline structure have different directions of helical axes becauseedge-shaped-structure parts are present in the layered-structure area inwhich the planes of nematic layers are layered. Therefore, thepolarized-light selective reflection layer reflects imaging light not byspecular reflection but by diffuse reflection, and the reflected lightthus can be well recognized as an image. At this time, as a result ofstructural non-uniformity in the cholesteric liquid crystallinestructure, the polarized-light selective reflection layer diffuses theselectively reflected light. The polarized-light selective reflectionlayer can, therefore, reflect a specific polarized light component whilediffusing it, and, at the same time, transmit the other light componentswithout diffusing them. For this reason, the environmental light andimaging light that pass through the polarized-light selective reflectionlayer do not undergo the above-described depolarization, and it is thuspossible to improve image visibility while maintaining thepolarized-light-separating property inherent in the polarized-lightselective reflection layer.

As mentioned above, according to the present invention, because of thepolarized-light-separating property of the cholesteric liquidcrystalline structure, the influence of environmental light such assunlight and light from lighting fixtures on the projection screen issuppressed, and image contrast is thus enhanced; on the other hand, as aresult of structural non-uniformity in the cholesteric liquidcrystalline structure, the projection screen diffuses, withoutdecreasing image visibility, imaging light when the projection screenreflects that light. For this reason, the projection screen of theinvention can sharply display an image even under bright environmentallight. Moreover, because the projection screen does not glare as aresult of structural non-uniformity in the cholesteric liquidcrystalline structure, it is not necessary separately to provide ananti-glaring layer or the like having a rough (matte) surface, and ahigh-quality, sharp image that gives no rough feel can be obtainedwithout an anti-glaring layer.

Further, according to the present invention, by allowing thepolarized-light selective reflection layer selectively to reflect lightin a specific wave range that covers only a part of the visible region,more specifically, by allowing the polarized-light selective reflectionlayer to have, for light in a wave range that covers only a part of thevisible region, reflectivity not less than half the maximum reflectivityof this layer, the influence of environmental light such as sunlight andlight from lighting fixtures on the projection screen is furthersuppressed. Therefore, image contrast is enhanced, and image visibilityis further improved.

A projector, such as a liquid crystal projector, that projects imaginglight on a projection screen attains color display by using light in thewave ranges of red (R), green (G), and blue (B) colors, the threeprimary colors. For example, assuming that light emitted from aprojector enters the projection screen vertically to it, light inselective reflection wave ranges whose centers are between 430 nm and460 nm, between 540 nm and 570 nm, and between 580 nm and 620 nm areprojected on the screen. Therefore, if the projection screen on whichsuch imaging light is projected is made selectively to reflect onlylight in the above-described wave ranges, the light in the wave rangesof the three primary colors, projected from a projector, is efficientlyreflected, while, of the environmental light such as sunlight and lightfrom lighting fixtures, the visible light that is not in theabove-described wave ranges is not reflected. It is thus possible toattain excellent color image display while enhancing image contrast.

Furthermore, according to the present invention, by controlling thestate of polarization of imaging light to be projected on the projectionscreen from a projector, it is possible to suppress the influence, onthe projection screen, of stray light originating from the imaginglight, thereby enhancing image contrast and further improving imagevisibility.

Namely, by making the imaging light that is projected on the projectionscreen from a projector contain mainly a polarized light componentidentical to the polarized light component that the projection screenselectively reflects, it is possible effectively to prevent productionof stray light or the like from a polarized light component (e.g.,left-handed circularly polarized light) that is different from thepolarized light component that the polarized-light selective reflectionlayer in the projection screen selectively reflects, thereby enhancingimage contrast.

Furthermore, according to the present invention, by controlling thestate of polarization of the light emitted from the illuminant, it ispossible to suppress the influence of this light on the projectionscreen, thereby enhancing image contrast and further improving imagevisibility.

Namely, in the case where the illuminant is so positioned that the lightemitted from the illuminant directly illuminates the projection screen,it is preferable to make the light emitted from the illuminant towardthe projection screen contain mainly a polarized light component (e.g.,left-handed circularly polarized light) that is different from thepolarized light component that the projection screen selectivelyreflects. By doing so, it is possible effectively to prevent the lightof the illuminant from being reflected from the polarized-lightselective reflection layer in the projection screen, thereby enhancingimage contrast. On the other hand, when the illuminant is so positionedthat the light emitted from the illuminant illuminates the projectionscreen indirectly via a reflector, it is preferable to make the lightemitted from the illuminant toward the reflector contain mainly apolarized light component (e.g., right-handed circularly polarizedlight) that is identical with the polarized light component that theprojection screen selectively reflects. If so made, the light from theilluminant, the state of polarization of the light being reversed by thereflector, is to contain mainly a polarized light component (e.g.,left-handed circularly polarized light) that is different from thepolarized light component that the projection screen selectivelyreflects. Therefore, the polarized-light selective reflection layer inthe projection screen does not reflect the light emitted from theilluminant, and image contrast is thus enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings,

FIG. 1 is a diagrammatic sectional view showing a projection screenaccording to an embodiment of the present invention;

FIGS. 2A and 2B are illustrations showing the state of orientation ofand optical function of the polarized-light selective reflection layerin the projection screen shown in FIG. 1;

FIG. 3 is a photomicrograph showing an example of the sectionalstructure of the cholesteric liquid crystalline structure of thepolarized-light selective reflection layer in the projection screenshown in FIG. 1;

FIG. 4 is an illustration showing in more detail the state oforientation of and optical function of the polarized-light selectivereflection layer in the projection screen shown in FIG. 1;

FIG. 5 is a photomicrograph showing another example of the sectionalstructure of the cholesteric liquid crystalline structure of thepolarized-light selective reflection layer in the projection screenshown in FIG. 1;

FIG. 6 is a diagrammatic sectional view showing a modification of theprojection screen shown in FIG. 1;

FIG. 7 is a diagrammatic sectional view showing another modification ofthe projection screen shown in FIG. 1;

FIG. 8 is a diagrammatic sectional view showing a further modificationof the projection screen shown in FIG. 1;

FIG. 9 is a diagrammatic sectional view showing a still furthermodification of the projection screen shown in FIG. 1;

FIG. 10 is a diagrammatic sectional view showing yet anothermodification of the projection screen shown in FIG. 1;

FIG. 11 is a diagrammatic sectional view showing another modification ofthe projection screen shown in FIG. 1;

FIG. 12 is a diagrammatic sectional view showing a further modificationof the projection screen shown in FIG. 1;

FIG. 13 is a diagrammatic view showing an example of a projection systemcomprising a projection screen according to an embodiment of the presentinvention; and

FIG. 14 is a diagrammatic view showing another example of a projectionsystem comprising a projection screen according to an embodiment of thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the present invention will be described hereinafter withreference to the accompanying drawings.

Projection Screen

First of all, a projection screen according to an embodiment of thepresent invention will be described with reference to FIG. 1.

As shown in FIG. 1, a projection screen 10 according to this embodimentis for displaying an image by reflecting imaging light projected fromthe viewer's side (the upper side of the figure), and includes apolarized-light selective reflection layer 11 having a cholestericliquid crystalline structure and adapted selectively to reflect aspecific polarized light component, and a substrate 12 that supports thepolarized-light selective reflection layer 11.

Of these components, the polarized-light selective reflection layer 11is made from a cholesteric liquid crystalline composition, andphysically, liquid crystalline molecules in this layer are aligned inhelical fashion in which the directors of the liquid crystallinemolecules are continuously rotated in the direction of the thickness ofthe layer.

As a result of such a physical alignment of the liquid crystallinemolecules, the polarized-light selective reflection layer 11 has thepolarized-light-separating property, the property of separating a lightcomponent circularly polarized in one direction from a light componentcircularly polarized in the opposite direction. Namely, thepolarized-light selective reflection layer 11 converts unpolarized lightthat enters this layer along the helical axis into light in twodifferent states of polarization (right-handed circularly polarizedlight and left-handed circularly polarized light), and transmits one ofthese lights and reflects the other. This phenomenon is known ascircular dichroism. If the direction of rotation of liquid crystallinemolecular helix is selected properly, a light component circularlypolarized in the same direction as this direction of rotation isreflected selectively.

In this case, the scattering of polarized light is maximized at thewavelength λ0 given by the following equation (1):λ0=nav·p,  (1)wherein p is the helical pitch in the helical structure consisting ofliquid crystalline molecules (the length of one liquid crystallinemolecular helix), and nav is the mean refractive index on a planeperpendicular to the helical axis.

On the other hand, the width Δλ of the wave range in which thewavelength of light to be reflected falls is given by the followingequation (2):Δλ=Δn·p,  (2)wherein Δn is the value of birefringence.

Namely, as shown in FIG. 1, of the unpolarized light that has enteredthe projection screen 10 from the viewer's side (i.e., right-handedcircularly polarized light 31R and left-handed circularly polarizedlight 31L in the selective reflection wave range, and right-handedcircularly polarized light 32R and left-handed circularly polarizedlight 32L not in the selective reflection wave range), one of thecircularly polarized light components in the wave range (selectivereflection wave range) with the width Δλ, centered at the wavelength λ0(e.g., right-handed circularly polarized light 31R in the selectivereflection wave range) is reflected from the projection screen 10 asreflected light 33, and the other light (e.g., left-handed circularlypolarized light 31L in the selective reflection wave range, andright-handed circularly polarized light 32R and left-handed circularlypolarized light 32L not in the selective reflection wave range) passthrough the projection screen 10.

The cholesteric liquid crystalline structure of such a polarized-lightselective reflection layer 11 has a plurality of helical-structure parts30 that have different directions of helical axes L, as shown in FIG.2A. As a result of structural non-uniformity in such a cholestericliquid crystalline structure, the light that the polarized-lightselective reflection layer 11 selectively reflects (reflected light 33)is diffused. The state in which the cholesteric liquid crystallinestructure is structurally non-uniform herein includes the state in whichthe helical-structure parts 30 contained in the cholesteric liquidcrystalline structure have different directions of helical axes L; thestate in which at least some of the planes of nematic layers (the planeson which the directors of liquid crystalline molecules point in the sameX-Y direction) are not parallel to the plane of the polarized-lightselective reflection layer 11 (the state in which, in a sectional TEMphoto of a cholesteric liquid crystalline structure specimen that hasbeen stained (a photomicrograph of the sectional structure taken by atransmission electron photomicroscope), continuous curves that appear aslight-and-dark patterns are not parallel to the substrate plane); andthe state in which finely divided particles of a cholesteric liquidcrystal are dispersed in the cholesteric liquid crystalline structure asa pigment. The “diffusion” that is caused by such structuralnon-uniformity in the cholesteric liquid crystalline structure meansthat the light (imaging light) reflected at the projection screen 10 isspread or scattered to such an extent that viewers can recognize thereflected light as an image. In this specification, the term“helical-structure part” refers to a block structure of liquidcrystalline molecules in which the helical axes L extend substantiallyin one direction and the helical length is substantially equal to one ormore helical pitches.

On the contrary, a conventional cholesteric liquid crystalline structureis in the sate of planar orientation, and the helical axes L inhelical-structure parts 30 contained in the cholesteric liquidcrystalline structure extend in parallel in the direction of thethickness of the layer, as shown in FIG. 2B. Therefore, when thecholesteric liquid crystalline structure selectively reflects light,specular reflection occurs (see reference numeral 36 in FIG. 2B).

The details of the cholesteric liquid crystalline structure of thepolarized-light selective reflection layer 11 will be describedhereinafter with reference to FIGS. 3 to 5.

FIG. 3 is a sectional TEM photo showing an example of the sectionalstructure of the cholesteric liquid crystalline structure of thepolarized-light selective reflection layer 11.

As shown in FIG. 3, the cholesteric liquid crystalline structure of thepolarized-light selective reflection layer 11 has a layered-structurearea in which the planes of nematic layers are layered andedge-shaped-structure parts formed in the layered-structure area by thepartial edge dislocation of the planes of nematic layers. In FIG. 3, theblack-and-white lines indicate the planes of nematic layers, and thedistance between a pair of the white and black lines is equal to onepitch. The direction of the helical axis is identical with the directionof the normal to these two lines.

The term “edge-shaped structure” herein refers to a liquid crystallinestructure formed as a result of edge dislocation, composed of linesconnecting defects (disclination) caused by abrupt changes in thedirection of the helical axis L in the cholesteric liquid crystallinestructure of the polarized-light selective reflection layer 11. Forexample, the structures shown in the circles in FIG. 3 are theedge-shaped structures. If such edge-shaped structures are formed in thecholesteric liquid crystalline structure, the directions of the helicalaxes L partially change in the edge-shaped-structure parts and theirvicinity. As a result, the above-described helical-structure parts 30that have different directions of helical axis L should exist in thecholesteric liquid crystalline structure. Because of the existence of aplurality of the helical-structure parts 30, light incident on thepolarized-light selective reflection layer 11 is reflected not byspecular reflection but by diffuse reflection, so that the reflectedlight can be well recognized as an image.

It is preferable that the edge-shaped-structure parts contained in thecholesteric liquid crystalline structure of the polarized-lightselective reflection layer 11 be of a predetermined density.Specifically, the number of the edge-shaped-structure parts present in across section of the polarized-light selective reflection layer 11 in apredetermined size (e.g., a cross section with a base length of 100 μmand a height of 1 μm) is preferably from 6 to 70, particularly from 10to 50. In the case where the number of the edge-shaped-structure partsis smaller than this range, the polarized-light selective reflectionlayer 11 cannot fully cause diffuse reflection, so that the visual fieldof the projection screen 10 narrows. On the other hand, when the numberof the edge-shaped-structure parts exceeds the above-described range,the polarized-light selective reflection layer 11 cannot efficientlyreflect light because of excessive structural disturbance, so that theimage displayed on the projection screen 10 gets darker.

The number of the edge-shaped-structure parts can be determined fromsuch a sectional TEM photo as is shown in FIG. 3, taken by atransmission electron photomicroscope. For example, by countingvisually, it is possible to obtain the number of theedge-shaped-structure parts present in a predetermined-sized area of thephotomicrograph (the size corresponding to a cross section of 100 μm×1μm). In the case where the thickness of the specimen used for thismeasurement is smaller than 1 μm, the number of theedge-shaped-structure parts obtained is multiplied by a predeterminednumber (a value for converting the thickness of the specimen to 1 μm).

To form the above-described edge-shaped-structure parts in thecholesteric liquid crystalline structure of the polarized-lightselective reflection layer 11, it is possible to use any method that candisturb the alignment of liquid crystalline molecules in the cholestericliquid crystalline structure. Examples of methods useful for thispurpose include: a method in which a material that exerts aligning powernot in one direction is used as the substrate 12 on which thepolarized-light selective reflection layer 11 is formed; a method inwhich the content of a surface-active agent or leveling agent usuallyincorporated in the polarized-light selective reflection layer 11 isproperly controlled; a method in which a polymerizable compound havingno aligning power is incorporated in the polarized-light selectivereflection layer 11; and any combination of these methods.

By allowing the edge-shaped-structure parts to exist in theabove-described manner, the plurality of helical-structure parts 30 thathave different directions of helical axes L are formed in thecholesteric liquid crystalline structure of the polarized-lightselective reflection layer 11. Preferably, the plurality ofhelical-structure parts 30 thus formed include, in one cross sectiontaken in the direction of the normal A, that is, in the direction of thethickness of the polarized-light selective reflection layer 11, boththose helical-structure parts 30(R) in which the helical axes L aretilted clockwise relative to the normal A (those helical-structure partsin which the helical axes L make acute angles α with the normal A in aclockwise direction) and those helical-structure parts 30(L) in whichthe helical axes L are tilted counterclockwise relative to the normal A(those helical-structure parts in which the helical axes L make acuteangles β with the normal A in a counterclockwise direction), as shown inFIG. 4. In this case, in some of the helical-structure parts 30, thedirections of the helical axes L may be the same as the direction of thenormal A (see reference numeral 30(V) in FIG. 4). If thehelical-structure parts 30 include the above-described two differenttypes of parts, the polarized-light selective reflection layer 11reflects the incident light not by specular reflection but by diffusereflection in two or more different directions, so that the reflectedlight can be well recognized as an image.

A sectional TEM photo of the cholesteric liquid crystalline structurewhose structure is as shown in FIG. 4 is shown in FIG. 5.

Specifically, in the cholesteric liquid crystalline structure of thepolarized-light selective reflection layer 11 shown in FIGS. 4 and 5, itis preferable that the angles between the helical axes L in thehelical-structure parts 30 and the normal A be from 0 to 45°,particularly from 0 to 30°. If these angles are greater than this range,the polarized-light selective reflection layer 11 cannot efficientlyreflect incident light toward the viewer's side.

Further, it is preferable that the helical-structure parts 30 in whichthe helical axes L make angles in the above-described range with thenormal A occupy 50% or more of the cholesteric liquid crystallinestructure of the polarized-light selective reflection layer 11 shown inFIGS. 4 and 5. This is because if the helical-structure parts 30 inwhich the helical axes L make angles in the above-described range withthe normal A occupy less than 50% of the cholesteric liquid crystallinestructure, the polarized-light selective reflection layer 11 cannotreflect light at high efficiency, so that the image displayed on theprojection screen 10 becomes darker.

The angle between the helical axis L in the helical-structure part 30and the normal A can be determined from such a sectional TEM photo as isshown in FIG. 5, taken by a transmission electron photomicroscope. Forexample, this angle can be determined by measuring the angle between thehelical axis L in the helical-structure part (the helical-structure partin which the helical length of liquid crystalline molecules is equal toone or more pitches) and the normal A in the photomicrograph. In FIG. 5,black-and-white lines indicate the planes of nematic layers, and thedistance between a pair of these lines is equal to one helical pitch.The direction of the helical axis L is equal to the direction of thenormal to these lines.

To form, in the cholesteric liquid crystalline structure of thepolarized-light selective reflection layer 11, the helical-structureparts 30 in which the angles between the helical axes L and the normal Aare in the above-described range, it is possible to use any method thatcan disturb the alignment of liquid crystalline molecules in thecholesteric liquid crystalline structure. Examples of methods usefulherein include: a method in which a material that exerts aligning powernot in one direction is used as the substrate 12 on which thepolarized-light selective reflection layer 11 is formed; a method inwhich the content of a surface-active agent or leveling agent usuallyincorporated in the polarized-light selective reflection layer 11 isproperly controlled; a method in which a polymerizable compound havingno aligning power is incorporated in the polarized-light selectivereflection layer 11; and any combination of these methods.

Preferably, the helical-structure parts 30 contained in the cholestericliquid crystalline structure of the polarized-light selective reflectionlayer 11 have specific helical pitches so that the polarized-lightselective reflection layer 11 can selectively reflect light in aspecific wave range that covers only a part of the visible region (e.g.,a wave range of 400 to 700 nm), that is, the polarized-light selectivereflection layer 11 has, for light in a wave range that covers only apart of the visible region (e.g., a wave range of 400 to 700 nm),reflectivity not less than the maximum reflectivity of this layer 11.More specifically, it is preferable that the cholesteric liquidcrystalline structure of the polarized-light selective reflection layer11 has two or more discontinuously different helical pitches so that thepolarized-light selective reflection layer 11 selectively reflects onlylight in a wave range that is identical to the wave range in whichimaging light projected from a projector such as a liquid crystalprojector falls. In general, a projector attains color display by usinglight in the wave ranges of red (R), green (G) and blue (B) colors, thethree primary colors. Therefore, assuming that light enters thepolarized-light selective reflection layer 11 vertically to it, it ispreferable to determine the helical pitches in the cholesteric liquidcrystalline structure so that the polarized-light selective reflectionlayer 11 selectively reflects light in wave ranges whose centers arebetween 430 nm and 460 nm, between 540 nm and 570 nm, and between 580 nmand 620 nm.

The wave ranges of 430 to 460 nm, 540 to 570 nm, and 580 to 620 nm thatare used as the red (R), green (G) and blue (B) color wave ranges,respectively, are wave ranges commonly used for color filters, lightsources, or the like for use in displays that produce white color by thethree primary colors. Red (R), green (G) and blue (B) colors are shownas line spectra maximized at specific wavelengths (e.g., in the case ofgreen (G) color, this wavelength is typically 550 nm). However, theseline spectra have certain widths, and moreover, the projected light haswavelengths that vary depending upon the design of the projector, thetype of the light source, and the like. It is, therefore, preferablethat the wave range for each color has a width of 30 to 40 nm. If thered (R), green (G) and blue (B) color wave ranges are set outside theabove-described respective ranges, it is impossible to produce purewhite, and only yellowish or reddish white is obtained.

In the case where the red (R), green (G) and blue (B) color wave rangesare set as selective reflection wave ranges that are independent of oneanother, it is preferable that the cholesteric liquid crystallinestructure of the polarized-light selective reflection layer 11 has threediscontinuously different helical pitches. There is a case where the red(R) and green (G) color wave ranges are included in the selectivereflection wave range corresponding to one helical pitch. In this case,it is preferable that the cholesteric liquid crystalline structure hastwo discontinuously different helical pitches.

To give two or more discontinuously different helical pitches in thecholesteric liquid crystalline structure of the polarized-lightselective reflection layer 11, the polarized-light selective reflectionlayer 11 may be formed by laminating two or more partial selectivereflection layers having different helical pitches. Specifically, asshown in FIG. 6, a partial selective reflection layer 11 a thatselectively reflects light in the blue (B) color wave range, a partialselective reflection layer 11 b that selectively reflects light in thegreen (G) color wave range, and a partial selective reflection layer 11c that selectively reflects light in the red (R) color wave range may besuccessively layered from the substrate 12 side. The order in which thepartial selective reflection layers 11 a, 11 b and 11 c are layered isnot necessarily limited to the above-described one. Each one of thepartial selective reflection layers 11 a, 11 b and 11 c shown in FIG. 6has a cholesteric liquid crystalline structure adapted selectively toreflect a specific polarized light component (e.g., right-handedcircularly polarized light), like the polarized-light selectivereflection layer 11 shown in FIGS. 1 and 2A. Moreover, as a result ofstructural non-uniformity in the cholesteric liquid crystallinestructure, each one of the partial selective reflection layer 11 a, 11 band 11 c diffuses the selectively reflected light.

It is preferable that the polarized-light selective reflection layer 11(or the partial selective reflection layers 11 a, 11 b and 11 cconstituting the polarized-light selective reflection layer 11) be madeto have such a thickness that the layer 11 selectively reflects nearly100% of the incident light in a specific state of polarization (such athickness that the reflectance is saturated). When the polarized-lightselective reflection layer 11 selectively reflects less than 100% of thespecific polarized light component (e.g., right-handed circularlypolarized light), the layer cannot efficiently reflect the imaginglight. Although the reflectance of the polarized-light selectivereflection layer 11 (or the partial selective reflection layers 11 a, 11b and 11 c constituting the polarized-light selective reflection layer11) depends directly on the number of helixes, the reflectance dependsindirectly on the thickness of the polarized-light selective reflectionlayer 11 if the helical pitch is fixed. Specifically, it is said thatapproximately 4 to 8 pitches are needed to obtain a reflectance of 100%.Therefore, the partial selective reflection layer 11 a, 11 b or 11 cthat reflects light in the red (R), green (G) or blue (B) color waverange is required to have a thickness of approximately 1 to 10 μm,although this thickness varies depending on the type of the componentsof the liquid crystalline composition used for forming this layer and onthe selective reflection wave range of this layer. On the other hand,the partial selective reflection layers 11 a, 11 b and 11 c should notbe of unlimited thickness because if these layers are excessively thick,it becomes difficult to control the orientation of these layers, thelayers cannot be made uniform, and the materials themselves for theselayers absorb light to a greater extent. For this reason, it isappropriate that each partial selective reflection layer 11 a, 11 b or11 c has a thickness in the above-described range.

In order to increase adhesion between the polarized-light selectivereflection layer 11 (or the partial selective reflection layers 11 a, 11b and 11 c constituting the polarized-light selective reflection layer)and the substrate 12, an adhesion-improving layer (intermediate layer)28 may be provided between these two layers, as shown in FIG. 7. Anytype of layer or any material is used for the adhesion-improving layer28, and acrylic or epoxy materials may be used, for example.

Next, the details of the substrate 12 will be described below.

The substrate 12 is for supporting the polarized-light selectivereflection layer 11, and a material selected from plastic films, metals,paper, cloth, glass, and the like can be used for forming the substrate12.

It is preferable that the substrate 12 be or contain a light-absorbinglayer adapted to absorb light in the visible region.

Specifically, for example, the substrate 12 (12A) may be made of aplastic film in which a black pigment is incorporated (e.g., a black PETfilm in which carbon is incorporated). In this case, the substrate 12itself can also serve as a light-absorbing layer (light-absorptivesubstrate). Therefore, of the unpolarized light entering the projectionscreen 10 from the viewer's side, those lights that are inherently notreflected from the projection screen 10 as reflected light 33 (e.g.,left-handed circularly polarized light 31L in the selective reflectionwave range, and right-handed circularly polarized light 32R andleft-handed circularly polarized light 32L not in the selectivereflection wave range) and the light that enters the projection screen10 from the backside are absorbed by the substrate 12. It is, therefore,possible effectively to prevent reflection of environmental light suchas sunlight and light from lighting fixtures and production of straylight from imaging light.

The substrate 12 is not limited to the above-described substrate 12(12A) shown in FIG. 8; the substrate may be such a substrate 12 (12B or12C) as is shown in FIG. 9 or 10, in which a light-absorbing layer 15containing a black pigment or the like is formed on one surface of atransparent base film 14 such as a plastic film.

To obtain a projection screen that can be rolled up, it is preferable tomake the thickness of the substrate 12 between 15 μm and 300 μm,particularly between 25 μm and 100 μm. On the other hand, if thesubstrate 12 is not necessarily required to have flexibility as in thecase where the resulting projection screen is used, for example, as apanel, there is no limitation on the thickness of the substrate 12.

Examples of plastic films that can be used as materials for thesubstrate 12 or base film 14 include films of such thermoplasticpolymers as polycarbonate polymers, polyester polymers includingpolyethylene terephthalate, polyimide polymers, polysulfone polymers,polyether sulfone polymers, polystyrene polymers, polyolefin polymersincluding polyethylene and polypropylene, polyvinyl alcohol polymers,cellulose acetate polymers, polyvinyl chloride polymers, polyacrylatepolymers, and polymethyl methacrylate polymers. Materials for thesubstrate 12 or base film 14 are not limited to the above-describedones, and materials such as metals, paper, cloth and glass also may beused.

Lamination of the polarized-light selective reflection layer 11 to thesubstrate 12 is usually conducted by applying a cholesteric liquidcrystalline composition to the substrate 12 and then subjecting theapplied layer to aligning treatment and curing treatment, as will bedescribed later.

In the above-described lamination process, it is necessary that thecholesteric liquid crystalline structure of the polarized-lightselective reflection layer 11 not be in the state of planar orientation.It is, therefore, preferable to use, as the substrate 12, a materialwhose surface to which the liquid crystalline composition will beapplied has no aligning power.

However, even when a material whose surface to which the liquidcrystalline composition will be applied has aligning power (e.g. astretched film) is used, the cholesteric liquid crystalline structure ofthe polarized-light selective reflection layer 11 can be made not in thestate of planar orientation if this surface of the material is subjectedin advance to surface treatment, the components of the liquidcrystalline composition are controlled, or the conditions under whichthe liquid crystalline composition is oriented are controlled.

Further, in the case where the surface of the substrate 12 to which theliquid crystalline composition will be applied has aligning power, suchan intermediate layer 13 as an adherent layer may be provided betweenthe polarized-light selective reflection layer 11 and the substrate 12(12A), as shown in FIG. 11. By providing such an intermediate layer 13,it is possible to control the orientation of the cholesteric liquidcrystalline structure of the polarized-light selective reflection layer11, and to make the directors of liquid crystalline molecules in thecholesteric liquid crystalline structure of the polarized-lightselective reflection layer 11, present in the vicinity of the surface ofthe intermediate layer 13, point in two or more different directions. Bythe use of the intermediate layer 13 (such as an adherent layer), it isalso possible to increase the adhesion between the polarized-lightselective reflection layer 11 and the substrate 12. Any material can beused for such an intermediate layer 13 as long as the material is highlyadherent to both the polarized-light selective reflection layer 11 andthe substrate 12; commercially available materials may be used. Specificexamples of such materials include a PET film with an adherent layer“A4100” manufactured by Toyobo Co., Ltd., Japan, and adherent materialssuch as “AC-X”, “AC-L” and “AC-W” manufactured by Panac Co., Ltd.,Japan. A black pigment or the like may be incorporated in theintermediate layer 13 so that the intermediate layer 13 can also serveas a light-absorbing layer 11 adapted to absorb light in the visibleregion, such as the substrate 12 (12A) shown in FIG. 8.

In the case where the surface of the substrate 12 has no aligning powerand the adhesion between the polarized-light selective reflection layer11 and the substrate 12 is satisfactorily high, it is not alwaysnecessary to provide the intermediate layer 13. To increase the adhesionbetween the polarized-light selective reflection layer 11 and thesubstrate 12, it is also possible to use a process-related method suchas corona discharge treatment or UV cleaning.

A process of producing the above-described projection screen 10 will bedescribed hereinafter.

The substrate 12 to which the polarized-light selective reflection layer11 will be laminated is firstly prepared. If necessary, such anintermediate layer 13 as an adherent layer is laminated to the surfaceof the substrate 12 on the side on which the polarized-light selectivereflection layer 11 will be provided. The surface of the substrate 12(or, if an intermediate layer 13 is present, the surface of this layer)to which a liquid crystalline composition will be applied is made tohave no aligning power.

Thereafter, a cholesteric liquid crystalline composition is applied tothe above-prepared substrate 12 and is then subjected to aligningtreatment and curing treatment, whereby the polarized-light selectivereflection layer 11 is laminated (fixed) to the substrate 12.

The steps (the steps of application, alignment and curing) forlaminating (fixing) the polarized-light selective reflection layer 11 tothe substrate 12 will be described in detail hereinafter.

(Step of Application)

In the step of application, a cholesteric liquid crystalline compositionis applied to the substrate 12 to form thereon a cholesteric liquidcrystal layer. Any of the known methods can be employed to apply theliquid crystalline composition to the substrate 12. Specifically, aroll, gravure, bar, slide, die, slit, or dip coating method can be usedfor this purpose. In the case where a plastic film is used as thesubstrate 12, a film coating method using a so-called roll-to-rollsystem may be used.

For the liquid crystalline composition that is applied to the substrate12, a chiral nematic liquid crystal or a cholesteric liquid crystal eachhaving a cholesteric regularity may be used. Although any liquidcrystalline material can be used as long as it can develop a cholestericliquid crystalline structure, a particularly preferable one forobtaining, after curing, an optically stable polarized-light selectivereflection layer 11 is a polymerizable liquid crystalline materialhaving polymerizable functional groups at both ends of its molecule.

Explanations will be given below with reference to the case where achiral nematic liquid crystal is used for the liquid crystallinecomposition. The chiral nematic liquid crystal is a mixture of apolymerizable, nematic liquid crystalline material and a chiral agent.The chiral agent herein refers to an agent for controlling the helicalpitch in the polymerizable, nematic liquid crystalline material to makethe resulting liquid crystalline composition cholesteric as a whole. Apolymerization initiator and other proper additives are added to theliquid crystalline composition.

Examples of polymerizable, nematic liquid crystalline materials includecompounds represented by the following general formulae (1) and (2-i) to(2-xi). These compounds may be used either singly or in combination.

In the above general formula (1), R¹ and R² independently represent ahydrogen atom or a methyl group. It is, however, preferable that both R¹and R² represent hydrogen atoms because a liquid crystalline compositioncontaining such a compound shows a liquid crystal phase at temperaturesin a wider range. X is any of hydrogen, chlorine, bromine, or iodineatoms, an alkyl group having 1 to 4 carbon atoms, methoxy group, cyanogroup or nitro group, preferably a chlorine atom or a methyl group.Further, in the above general formula (1), a and b that denote the chainlengths of the alkylene groups that serve as spacers between the(meth)acryloyloxy groups on both ends of the molecule and the aromaticrings are independently an integer of 2 to 12, preferably an integer of4 to 10, more preferably an integer of 6 to 9. Those compoundsrepresented by the general formula (1) in which a=b=0 are unstable,easily undergo hydrolysis, and have high crystallinity. On the otherhand, those compounds represented by the general formula (1) in which aand b are independently an integer of 13 or more have low isotropictransition temperatures (TI's). Because these compounds show liquidcrystal phases at temperatures in narrow ranges, they are undesirable.

Although a polymerizable liquid crystal monomer is, in the abovedescription, used as the polymerizable, nematic liquid crystallinematerial, it is also possible to use, as the polymerizable, nematicliquid crystal material, a polymerizable liquid crystal oligomer orpolymer, a liquid crystal polymer, or the like, properly selected fromconventionally proposed ones.

On the other hand, the chiral agent is a low molecular weight compoundcontaining an optically active site, having usually a molecular weightof not more than 1,500. The chiral agent is used in order to convert thepositive mono-axially-nematic structure of a polymerizable, nematicliquid crystalline material into a helical structure. Any type of lowmolecular weight compounds may be used as the chiral agent as long as itis compatible with the polymerizable, nematic liquid crystallinematerial in the state of solution or melt and can make the liquidcrystalline structure helical without impairing the liquid crystallinityof the material.

The chiral agent that is used for making the structure of the liquidcrystal helical is required to have any type of chirality at least inits molecule helical is required to have any type of chirality at leastin its molecule. Examples of chiral agents useful herein include thosecompounds having at least one asymmetric carbon atom; those compoundshaving asymmetric centers on hetero atoms, such as chiral amines orsulfoxides; and those axially chiral compounds having optically activesites, such as cumulene and binaphthol. More specific examples of chiralagents include commercially available chiral nematic liquid crystalssuch as a chiral dopant liquid crystal “S-811” manufactured by MerckKGaA.

However, depending on the nature of the chiral agent selected, thefollowing problems can occur: the nematic state of the polymerizable,nematic liquid crystalline material is destroyed, and the polymerizable,nematic liquid crystalline material loses its alignability; and, if thechiral agent is a non-polymerizable one, the liquid crystallinecomposition has reduced hardenability, and the cured film is poor inreliability. Moreover, the use of a large amount of a chiral agentcontaining an optically active site increases the cost of the liquidcrystalline composition. Therefore, to form a polarized-light selectivereflection layer having a cholesteric structure with a short helicalpitch, it is preferable to select, as theoptically-active-site-containing chiral agent to be incorporated in theliquid crystalline composition, a chiral agent whosehelical-structure-developing action is great. Specifically, it ispreferable to use one of the compounds represented by the followinggeneral formulae (3), (4) and (5), which are low-molecular-weightcompounds whose molecules are axially chiral.

In the above general formulae (3) and (4), R⁴ represents a hydrogen atomor a methyl group; Y is one of the above-enumerated groups (i) to(xxiv), preferably (i), (ii), (iii), (v) or (vii); and c and d thatdenote the chain lengths of the alkylene groups are independently aninteger of 2 to 12, preferably an integer of 4 to 10, more preferably aninteger of 6 to 9. Those compounds represented by the above generalformulae (3) and (4) in which c or d is 0 or 1 are poor in stability,easily undergo hydrolysis, and have high crystallinity. On the otherhand, those compounds represented by the general formulae (3) and (4) inwhich c or d is 13 or more have low melting points (Tm's). Thesecompounds are less compatible with the polymerizable, nematic liquidcrystalline material, so that a liquid crystalline compositioncontaining such a compound as the chiral agent may cause phaseseparation depending on the concentration of the compound.

The chiral agent is not necessarily polymerizable. However, if thechiral agent is polymerizable, it is polymerized with the polymerizable,nematic liquid crystalline material to give a stably fixed cholestericstructure. Therefore, from the viewpoint of thermal stability and thelike, it is desirable that the chiral agent be polymerizable. Inparticular, the use of a chiral agent having polymerizable functionalgroups at both ends of its molecule is preferable to obtain apolarized-light selective reflection layer 11 excellent in heatresistance.

The content of the chiral agent in the liquid crystalline composition isoptimally decided in consideration of the helical-structure-developingability of the chiral agent, the cholesteric liquid crystallinestructure of the resulting polarized-light selective reflection layer11, and so forth. Although the amount of the chiral agent to be addedgreatly varies depending upon the components of the liquid crystallinecomposition, that amount is from 0.01 to 60 parts by weight, preferablyfrom 0.1 to 40 parts by weight, more preferably from 0.5 to 30 parts byweight, most preferably from 1 to 20 parts by weight, for 100 parts byweight of the liquid crystalline composition. In the case where theamount of the chiral agent added is smaller than this range, there is apossibility that the liquid crystalline composition cannot fully becomecholesteric. On the other hand, when the amount of the chiral agentadded exceeds the above-described range, the alignment of liquidcrystalline molecules is impeded, and this undesired amount mayadversely affect the liquid crystalline composition in the course ofcuring with activating radiation or the like.

Although the liquid crystalline composition can be applied as it is tothe substrate 12, the composition may be dissolved in a suitable solventsuch as an organic solvent to give an ink in order to make the viscosityof the liquid crystalline composition fit for an applicator or attainexcellent alignment of liquid crystalline molecules.

Although any solvent can be used for the above purpose as long as it candissolve the above-described polymerizable liquid crystalline material,it is preferable that the solvent does not attack the substrate 12,Specific examples of solvents useful herein include acetone,3-methoxy-butyl acetate, diglyme, cyclohexanone, tetrahydrofuran,toluene, xylene, chlorobenzene, methylene chloride, and methyl ethylketone. The polymerizable liquid crystalline material may be diluted toany degree. However, considering that a liquid crystal itself is amaterial having low solubility and high viscosity, it is preferable todilute the polymerizable liquid crystalline material to such a degreethat the content of the liquid crystalline material in the dilutedsolution is in the order of preferably 5 to 50%, more preferably 10 to30%.

(Step of Alignment)

After applying the liquid crystalline composition to the substrate 12 toform thereon a cholesteric liquid crystal layer 11 in theabove-described step of application, the cholesteric liquid crystallayer is, in the step of alignment, held at a predetermined temperatureat which the cholesteric liquid crystal layer develops a cholestericliquid crystalline structure, thereby aligning liquid crystallinemolecules in the cholesteric liquid crystal layer.

The cholesteric liquid crystalline structure of the polarized-lightselective reflection layer 11 that should be finally obtained in thisembodiment is one not in the state of planar orientation but in such astate of orientation as is shown in FIGS. 2A, 3, 4 and 5, in which aplurality of helical-structure parts 30 that have different directionsof helical axes L are present because of the existence of edge-shapedstructures. Even so, it is necessary to conduct alignment treatment.Namely, although it is not necessary to align, in one direction on thesubstrate 12, the directors of liquid crystalline molecules in thecholesteric liquid crystalline structure, it is necessary to conduct analignment treatment such that the plurality of helical-structure parts30 are formed in the cholesteric liquid crystalline structure.

When the cholesteric liquid crystal layer formed on the substrate 12 isheld at a predetermined temperature at which the cholesteric liquidcrystal layer develops a cholesteric liquid crystalline structure, thelayer shows a liquid crystal phase. At this time, as a result of to theself-accumulating action of liquid crystalline molecules themselves,continuous rotation of the directors of the liquid crystalline moleculesoccurs in the direction of the thickness of the layer, and a helicalstructure is produced. It is possible to fix this cholesteric liquidcrystalline structure that is in a liquid crystal phase state by curingthe cholesteric liquid crystal layer with a technique that will bedescribed later.

In the case where the liquid crystalline composition applied to thesubstrate 12 contains a solvent, the step of alignment is usuallyconducted along with drying treatment for removing the solvent. Thedrying temperature suitable for removing the solvent is from 40 to 120°C., preferably from 60 to 100° C. Any drying time (heating time) will doas long as a cholesteric liquid crystalline structure is developed andsubstantially all of the solvent is removed. For example, the dryingtime (heating time) is preferably from 15 to 600 seconds, morepreferably from 30 to 180 seconds. After once conducting the dryingtreatment, if it is realized that the liquid crystal layer is not fullyorientated, this layer may be further heated accordingly. In the casewhere a vacuum drying technique is used in this drying treatment, it ispreferable separately to conduct heat treatment in order to align liquidcrystalline molecules.

(Step of Curing)

After aligning liquid crystalline molecules in the cholesteric liquidcrystal layer in the above-described step of alignment, the cholestericliquid crystal layer is cured in the step of curing, thereby fixing thecholesteric liquid crystalline structure that is in the liquid crystalphase state.

To effect the step of curing, it is possible to use: (1) a method inwhich the solvent contained in the liquid crystalline composition isevaporated; (2) a method in which liquid crystalline molecules in theliquid crystalline composition are thermally polymerized; (3) a methodin which liquid crystalline molecules in the liquid crystallinecomposition are polymerized by the application of radiation; or (4) anycombination of these methods.

Of the above methods, method (1) is suitable for the case where a liquidcrystal polymer is used as the polymerizable, nematic liquid crystallinematerial that is incorporated in the liquid crystalline composition forforming the cholesteric liquid crystal layer. In this method, the liquidcrystal polymer is dissolved in a solvent such as an organic solvent,and this solution is applied to the substrate 12, In this case, asolidified, cholesteric liquid crystal layer can be obtained by simplyremoving the solvent by drying. The type of the solvent, the dryingconditions, and so on are the same as those ones that are used in theaforementioned steps of application and alignment.

The above-described method (2) is for curing the cholesteric liquidcrystal layer by thermally polymerizing by heating liquid crystallinemolecules in the liquid crystalline composition. In this method, thestate of bonding of the liquid crystalline molecules varies according tothe heating (baking) temperature. Therefore, if the liquid crystal layeris heated unevenly, the cured layer cannot be uniform in physicalproperties such as film hardness and in optical properties. In order tolimit variations in film hardness to ±10%, it is preferable to controlthe heating temperature so that it varies only within ±5%, particularly±2%.

Any method may be employed to heat the cholesteric liquid crystal layerformed on the substrate 12 as long as the method can provide uniformityin heating temperature. The liquid crystal layer may be placed directlyon a hot plate and held as it is, or placed indirectly on a hot platewith a thin air layer interposed between the liquid crystal layer andthe hot plate and held in parallel with the hot plate. Moreover, amethod using a heater capable of heating the whole of a particularspace, such as an oven, may be employed, where the liquid crystal layeris placed in or passed through such a heater. If a film coater or thelike is used, it is preferable to make the drying zone long enough tomake the heating time sufficiently long.

The required heating temperature is usually as high as 100° C. or more.However, considering the heat resistance of the substrate 12, it ispreferable to limit this temperature to below approximately 150° C. If aspecialized film or the like having significantly high heat resistanceis used as the substrate 12, the heating temperature can be made as highas 150° C. or more.

The above-described method (3) is for curing the cholesteric liquidcrystal layer by photo-polymerizing liquid crystalline molecules in theliquid crystalline composition by the application of radiation. In thismethod, electron beams, ultraviolet rays, or the like suitable for theconditions can be used as the radiation source. In general, ultravioletlight is preferred because of the simplicity of ultraviolet lightirradiation systems. The wavelength of ultraviolet light useful hereinis from 250 to 400 nm. If ultraviolet light is used, it is preferable toincorporate a photopolymerization initiator in the liquid crystallinecomposition in advance.

Examples of photopolymerization initiators that can be incorporated inthe liquid crystalline composition include benzyl (bibenzoyl), benzoinisobutyl ether, benzoin isopropyl ether, benzophenone, benzoyl benzoicacid, benzoyl methylbenzoate, 4-benzoyl-4′-methyldiphenylsulfide,benzylmethyl ketal, dimethylamino-methyl benzoate,2-n-butoxyethyl-4-dimethylaminobenzoate, isoamylp-dimethylaminobenzoate, 3,3′-dimethyl-4-methoxybenzophenone,methyl-benzoyl formate,2-methyl-1-(4-(methylthio)phenyl)-2-morpholinopropan-1-one,2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butan-1-one,1-(4-dodecylphenyl)-2-hydroxy-2-methylpropan-1-one, 1-hydroxycyclo-hexylphenyl ketone, 2-hydroxy-2-methyl-1-phenylpropan-1-one,1-(4-isopropylphenyl)-2-hydroxy-2-methylpropan-1-one,2-chlorothioxanthone, 2,4-diethylthioxanthone,2,4-diisopropylthioxanthone, 2,4-dimethylthio-xanthone,isopropylthioxanthone, and 1-chloro-4-propoxythioxanthone. In additionto the photopolymerization initiator, sensitizers and leveling agentsmay be added to the liquid crystalline composition unless they hinderthe attainment of the object of the present invention.

The amount of the photopolymerization initiator to be added to theliquid crystalline composition is from 0.01 to 20% by weight, preferablyfrom 0.1 to 10% by weight, more preferably from 0.5 to 5% by weight, ofthe liquid crystalline composition.

By varying the contents of the above-described components in the liquidcrystalline composition, the helical-structure parts 30 that havedifferent directions of helical axes L as a result of the existence ofthe edge-shaped structures and the like can be formed in the cholestericliquid crystalline structure of the polarized-light selective reflectionlayer 11. Specifically, for example, a large amount of a surface-activeagent may be incorporated in the liquid crystalline composition todisturb the alignment of liquid crystalline molecules on the surface ofthe cholesteric liquid crystal structure; and a large amount of aphotopolymerization initiator may be incorporated in the liquidcrystalline composition to make the chain lengths of liquid crystallinemolecules in the cholesteric liquid crystalline structure short. In thelatter case, the photopolymerization initiator remaining even after thecompletion of reaction serves as an impurity that disturbs the alignmentof the liquid crystalline molecules in the cholesteric liquidcrystalline structure. Further, the alignment of the liquid crystallinemolecules in the cholesteric liquid crystalline structure may also bedisturbed by the addition of a polymerizable compound having no liquidcrystalline orientation to the liquid crystalline composition.Furthermore, the orientation of the cholesteric liquid crystal may bedisturbed by the addition of finely divided particles to the liquidcrystalline composition. The alignment of the liquid crystallinemolecules in the cholesteric liquid crystalline structure may also bedisturbed by the combined use of the above methods. The types andamounts of these additives can be properly selected depending on thepurpose of addition of the additives.

A projection screen 10 including the polarized-light selectivereflection layer that is composed of a single cholesteric liquid crystallayer can be obtained by conducting a series of the above-describedsteps (the steps of application, alignment and curing). It is alsopossible to obtain a projection screen 10 having a polarized-lightselective reflection layer 11 that contains a plurality of cholestericliquid crystal layers by repeating a series of the above-describedsteps. Namely, by repeating the above-described steps, it is possible toproduce a projection screen 10 in which a partial selective reflectionlayer 11 a that selectively reflects light in the blue (B) color waverange, a partial selective reflection layer 11 b that selectivelyreflects light in the green (G) color wave range, and a partialselective reflection layer 11 c that selectively reflects light in thered (R) color wave range are layered successively from the substrate 12side to constitute a polarized-light selective reflection layer, asshown in FIG. 6.

In the process of making such a multi-layered, polarized-light selectivereflection layer 11, as long as the underlying cholesteric liquidcrystal layer has been solidified, a liquid crystalline composition forforming the next cholesteric liquid crystal layer can be applied byusing the same technique as in the formation of the first liquid crystallayer. In this case, there is continuity between the cholesteric liquidcrystalline structure (the state of orientation) of the uppercholesteric liquid crystal layer and the cholesteric liquid crystallinestructure of the lower cholesteric liquid crystal layer. It is,therefore, unnecessary to provide an alignment-controlling layer or thelike between these two cholesteric liquid crystal layers. However, anintermediate layer such as an adherent layer may be provided betweenthese two cholesteric liquid crystal layers, as needed. The conditionsfor the steps of application, alignment, and curing in the formation ofthe second and later cholesteric liquid crystal layers and the materialsfor forming these liquid crystal layers are as mentioned above, so thatexplanations for them are omitted here.

Thus, according to this embodiment, the projection screen 10 includesthe polarized-light selective reflection layer 11 having a cholestericliquid crystalline structure and adapted selectively reflect a specificpolarized light component and to diffuse the selectively reflected lightas a result of structural non-uniformity in the cholesteric liquidcrystalline structure.

Due to the polarized-light-separating property of the cholesteric liquidcrystalline structure, the polarized-light selective reflection layer 11selectively reflects only a specific polarized light component (e.g.,right-handed circularly polarized light), so that the layer can be madeto reflect only approximately 50% of the unpolarized environmental lightsuch as sunlight and light from light fixtures that are incident on thislayer. For this reason, while retaining the brightness of thelight-indication part such as a white-indication part, it is possible tomake the brightness of the dark-indication part such as ablack-indication part nearly half, thereby enhancing image contrast toapproximately twice. In this case, if the imaging light to be projectedis made to contain mainly a polarized light component (e.g.,right-handed circularly polarized light) that is identical to thepolarized light component that the polarized-light selective reflectionlayer 11 selectively reflects, the polarized-light selective reflectionlayer can reflect nearly 100% of the imaging light projected on thislayer, that is, the polarized-light selective reflection layer 11 canefficiently reflect the imaging light.

Further, the polarized-light selective reflection layer 11 has acholesteric liquid crystalline structure that is structurallynon-uniform; this cholesteric liquid crystalline structure contains thehelical-structure parts 30 that have different directions of helicalaxes L and that are formed because of, for example, the existence of theedge-shaped-structure parts in the layered-structure area in which theplanes of nematic layers are layered. Therefore, the polarized-lightselective reflection layer 11 reflects the imaging light not by specularreflection but by diffuse reflection, so that the reflected light can bewell recognized as an image. At this time, because of structuralnon-uniformity in the cholesteric liquid crystalline structure, thepolarized-light selective reflection layer 11 diffuses the selectivelyreflected light. Therefore, while diffuse-reflecting a specificpolarized light component (e.g., right-handed circularly polarized light31R in the selective reflection wave range), the polarized-lightselective reflection layer 11 transmits the other light components(e.g., left-handed circularly polarized light 31L in the selectivereflection wave range, and right-handed circularly polarized light 32Rand left-handed circularly polarized light 32L not in the selectivereflection wave range). For this reason, the environmental light and theimaging light that pass through the polarized-light selective reflectionlayer do not undergo the previously-mentioned “depolarization.” It isthus possible to improve image visibility while retaining thepolarized-light-separating property of the polarized-light selectivereflection layer 11.

In the projection screen 10 according to this embodiment, alight-reflecting layer 16 for reflecting light incident on the substrate12 may be provided on the side of the substrate 12 opposite to the sideon which the polarized-light selective reflection layer 11 is provided,as shown in FIG. 12. If the substrate 12 contains a light-absorbinglayer in the manner shown in FIGS. 8 to 10, environmental light such assunlight and light from lighting fixtures that enter the projectionscreen 10 from the backside are effectively reflected before reachingthe substrate 12 (especially the light-absorbing layer contained in thesubstrate 12), so that it is possible effectively to suppress heatgeneration of the substrate 12, Preferable examples of materials thatcan be used as the light-reflecting layer 16 include white-coloredscattering layers (paper, white-colored films, coatings, etc.), metallicplates, and films made from aluminum powder.

Further, as shown in FIG. 12, a pressure sensitive adhesive layer 17useful for affixing, to an external member, the substrate 12 on whichthe polarized-light selective reflection layer 11 is formed may beprovided on the side of the substrate 12 opposite to the side on whichthe polarized-light selective reflection layer 11 is provided (on thebackside of the light-reflecting layer 16 in FIG. 12). If a pressuresensitive adhesive layer 17 is so provided, the projection screen 10 canbe affixed to an external member such as a white board or wall, ifnecessary. The pressure sensitive adhesive layer 17 is preferably alayer that can make the substrate 12 on which the polarized-lightselective reflection layer 11 is provided separably adhere to anexternal member. It is, therefore, preferable to use, as the pressuresensitive adhesive layer 17, a pressure sensitive adhesive film withslight tackiness such as a removable, pressure sensitive adhesive filmmanufactured by Panac Co., Ltd., Japan. Moreover, it is preferable tocover the surface of the pressure sensitive adhesive layer 17 with areleasing film 18 in order to protect the pressure sensitive adhesivelayer 17 before use.

Furthermore, as shown in FIG. 12, a functional layer 19 may be providedon the viewer's side surface of the polarized-light selective reflectionlayer 11. A variety of layers including hard coat (HC) layers,anti-glaring (AG) layers, anti-reflection (AR) layers,ultraviolet-absorbing (UV-absorbing) layers, and antistatic (AS) layerscan be used as the functional layer 19.

The hard coat layer is for preventing the surface of the projectionscreen 10 from being scratched or stained. The anti-glaring layer is forpreventing the projection screen 10 from glaring. The anti-reflectionlayer is for preventing the projection screen 10 from reflecting light.The ultraviolet-absorbing layer is for absorbing ultraviolet light thatenters the projection screen 10. The antistatic layer is for preventingthe projection screen 10 from being electrostatically charged. In thecase where the antistatic layer is used as the functional layer 19, thislayer is not necessarily provided on the viewer's side surface of thepolarized-light selective reflection layer 11 and can be provided on theback surface of the substrate 12. Moreover, carbon particles or the likemay be incorporated in the substrate 12 so that the substrate 12 itselfhas the function of eliminating static electricity.

The hard coat, anti-glaring, anti-reflection, ultraviolet-absorbing andantistatic layers, as the functional layers 19, will be described indetail hereinafter.

(Hard Coat Layer)

The hard coat layer is a member for preventing the surface of theprojection screen 10 from being scratched or stained and is provided onthe outermost, viewer's side surface of the polarized-light selectivereflection layer 11. The hard coat layer can have any surface hardnessas long as the layer can protect the projection screen 10 from damagessuch as scratches. It is, however, preferable that the surface hardnessof the hard coat layer expressed by the pencil hardness according to JISK5400 be 2H or more, particularly 4H or more. As long as the surfacehardness is in this range, the hard coat layer can satisfactorilyimprove the scratch resistance of the projection screen 10 and protectthe surface of the projection screen 10 from scratches at the time whenthe projection screen 10 is rolled up for storage.

In addition to the function of preventing the surface of the projectionscreen 10 from being scratched, the hard coat layer that is provided onthe outermost, viewer's side surface of the polarized-light selectivereflection layer 11 may have other functions. It is preferable thatthese other functions include at least one of the following functions:the function of preventing the projection screen 10 from glaring; thefunction of preventing the projection screen 10 from reflecting light;the function of absorbing ultraviolet light that enters the projectionscreen 10; and the function of preventing the projection screen 10 frombeing electrostatically charged. In other words, in the case whereprovided on the outermost, viewer's side surface of the polarized-lightselective reflection layer 11 are an anti-glaring layer for preventingthe projection screen 10 from glaring, an anti-reflection layer forpreventing the projection screen 10 from reflecting light, anultraviolet-absorbing layer for absorbing ultraviolet light that entersthe projection screen 10, and an antistatic layer for preventing theprojection screen 10 from being electrostatically charged, if thefunction of protecting the surface of the projection screen 10 isimparted to these layers by controlling their hardness, there can beobtained: a hard coat layer having the function of preventing glaring; ahard coat layer having the function of preventing reflection of light; ahard coat layer having the function of absorbing ultraviolet light; anda hard coat layer having the function of preventing staticelectrification.

To the hard coat layer, the function of absorbing ultraviolet light andthe function of preventing static electrification may be impartedtogether with either the function of preventing glaring or the functionof preventing reflection of light. Examples of hard coat layers havingthese functions are as follows: (1) a hard coat layer having thefunction of preventing glaring and the function of absorbing ultravioletlight; (2) a hard coat layer having the function of preventingreflection of light and the function of absorbing ultraviolet light; (3)a hard coat layer having the function of preventing glaring and thefunction of preventing static electrification; (4) a hard coat layerhaving the function of preventing reflection of light and the functionof preventing static electrification; (5) a hard coat layer having thefunction of preventing glaring, the function of absorbing ultravioletlight and the function of preventing static electrification; and (6) ahard coat layer having the function of preventing reflection of light,the function of absorbing ultraviolet light and the function ofpreventing static electrification.

By thus imparting other functions to the hard coat layer, there can beefficiently obtained a high-quality projection screen 10 having a simplestructure.

Examples of materials useful for forming such a hard coat layer includethermosetting resins, thermoplastic resins, ultraviolet-curing resins,electron-beam-curing resins, and two-part resins. Of these,ultraviolet-curing resins are preferred because it is possible to formeasily hard coat layers from such resins by simply conducting curingtreatment with ultraviolet light.

Examples of ultraviolet-curing resins useful herein include polyester,acrylic, urethane, amide, silicone and epoxy monomers, oligomers andpolymers. Of these, urethane monomers, oligomers and polymers arepreferred. More specifically, ultraviolet-curing resins havingultraviolet-polymerizable functional groups, especially thoseultraviolet-curing resins having 2 or more, particularly 3 to 6,ultraviolet-polymerizable functional groups in one molecule arepreferred.

Finely divided particles may be incorporated in the hard coat layer inorder to control its hardness. Any of various transparent materials suchas metallic oxides, glass and plastics can be used as the finely dividedparticles without limitation. Specific examples of finely dividedparticles useful herein include: electrically-conductive, inorganicfinely divided particles such as silica, alumina, titania, zirconia,calcium oxide, tin oxide, indium oxide, cadmium oxide and antimonyoxide; cross-linked or non-cross-linked, organic finely dividedparticles made from various polymers such as polymethyl methacrylate,polystyrene, polyurethane, acryl-styrene copolymers, benzoguanamine,melamine and polycarbonate; and silicone finely divided particles.

The finely divided particles may be in any shape: they may be inspherical bead shape or in amorphous powdery shape. One type, or two ormore types, of finely divided particles selected properly may be used.The mean particle diameter of the finely divided particles is between 1μm and 10 μm, preferably between 2 μm and 5 μm. In the finely dividedparticles, ultrafine particles of metallic oxides or the like may bedispersed or incorporated in order to control refractive index or toimpart electrical conductivity.

The content of the finely divided particles in the hard coat layer isproperly determined taking into consideration the mean particle diameterof the finely divided particles, the thickness of the hard coat layer,and so forth. In general, however, the content of the finely dividedparticles is preferably from 1 to 20 parts by weight, particularly from5 to 15 parts by weight, for 100 parts by weight of the resin that isused for the formation of the hard coat layer.

In addition to the finely divided particles, other additives such asphotopolymerization initiators, leveling agents, thixotropic agents,ultraviolet light absorbers, and antistatic agents may be incorporatedin the hard coat layer.

A method usually employed to form such a hard coat layer is as follows:the above-described materials are dissolved or dispersed in a propersolvent to give a hard-coat-layer-forming coating liquid, and thiscoating liquid is applied to the polarized-light selective reflectionlayer 11 formed on the substrate 12 and then dried and cured. Examplesof solvents for use in the hard-coat-layer-forming coating liquidinclude toluene, ethyl acetate, butyl acetate, methyl ethyl ketone,methyl isobutyl ketone, isopropyl alcohol, and ethyl alcohol. To applythe hard-coat-layer-forming coating liquid, any one of the known methodsmay be used. Specifically, a roll, gravure, bar, slide, die, slit or dipcoating method can be employed. Further, in the case where a plasticfilm is used as the substrate 12, a film coating method using aso-called roll-to-roll system may be used.

The thickness of the hard coat layer is preferably between 0.1 μm and100 μm, particularly between 1 μm and 10 μm. When the hard coat layerhas a thickness smaller than this range, there is a possibility that thefunction of preventing the surface of the projection screen 10 frombeing scratched cannot be satisfactorily obtained, and, if the hard coatlayer also has other functions such as the function of preventingreflection of light, these functions may not be fully obtained. On theother hand, when the hard coat layer has a thickness greater than theabove-described range, although these functions can be satisfactorilyobtained, there is a possibility that the hard coat layer impedestransmission of imaging light projected from a projector to lowerbrightness.

(Anti-Glaring Layer)

The anti-glaring layer is a member for preventing the projection screen10 from glaring and is provided on the viewer's side of thepolarized-light selective reflection layer 11. It is possible to providethe anti-glaring layer by roughening the surface of the hard coat layerformed in the above-described manner, for example.

(Anti-Reflection Layer)

The anti-reflection layer is a member for preventing the projectionscreen 10 from reflecting light and is provided on the viewer's side ofthe polarized-light selective reflection layer 11. It is possible toprovide the anti-reflection layer by subjecting the surface of the hardcoat layer formed in the above-described manner to treatment forimparting, to this surface, the property of preventing reflection ofextraneous light, for example.

(Ultraviolet-Absorbing Layer)

The ultraviolet-absorbing layer is a member for absorbing ultravioletlight that enters the projection screen 10 and is provided on theviewer's side of the polarized-light selective reflection layer 11. Theultraviolet-absorbing layer contains an ultraviolet light absorber andcan prevent the polarized-light selective reflection layer 11 from beingadversely affected by ultraviolet light that enters the projectionscreen 10. Specifically, for example, in the case where thepolarized-light selective reflection layer 11 is made from a cholestericliquid crystalline composition, the ultraviolet-absorbing layer canprevent the liquid crystalline composition from yellowing that occurswhen affected by ultraviolet light, and the polarized-light selectivereflection layer 11 can thus have improved weatherability. It ispreferable that the ultraviolet-absorbing layer absorbs 90% or more,more preferably 95% or more, of the ultraviolet light that enters theprojection screen 10. As long as the ultraviolet-absorbing layer canabsorb ultraviolet light to such an extent, the polarized-lightselective reflection layer 11 can show excellent weatherability.

In addition to the function of absorbing ultraviolet light that entersthe projection screen 10, the ultraviolet-absorbing layer that isprovided on the viewer's side of the polarized-light selectivereflection layer 11 may have other functions. It is preferable thatthese other functions include at least one of the following functions:the function of preventing the surface of the projection screen 10 frombeing scratched; the function of preventing the projection screen 10from glaring; the function of preventing the projection screen 10 fromreflecting light; and the function of preventing the projection screen10 from being electrostatically charged. In other words, in the casewhere provided on the viewer's side of the polarized-light selectivereflection layer 11 are a hard coat layer for preventing the surface ofthe projection screen 10 from being scratched, an anti-glaring layer forpreventing the projection screen 10 from glaring, an anti-reflectionlayer for preventing the projection screen 10 from reflecting light, andan antistatic layer for preventing the projection screen 10 from beingelectrostatically charged, if an ultraviolet light absorber isincorporated in these layers, there can be obtained: anultraviolet-absorbing layer having the function of protecting thesurface of the projection screen 10; an ultraviolet-absorbing layerhaving the function of preventing glaring; an ultraviolet-absorbinglayer having the function of preventing reflection of light; and anultraviolet-absorbing light layer having the function of preventingstatic electrification.

To such an ultraviolet-absorbing layer, the function of protecting thesurface of the projection screen 10 and the function of preventingstatic electrification may be imparted together with either the functionof preventing glaring or the function of preventing reflection of light.Examples of ultraviolet-absorbing layers having these functions are asfollows: (1) an ultraviolet-absorbing layer having the function ofprotecting the surface of the projection screen 10 and the function ofpreventing glaring; (2) an ultraviolet-absorbing layer having thefunction of protecting the surface of the projection screen 10 and thefunction of preventing reflection of light; (3) an ultraviolet-absorbinglayer having the function of preventing glaring and the function ofpreventing static electrification; (4) an ultraviolet-absorbing layerhaving the function of preventing reflection of light and the functionof preventing static electrification; (5) an ultraviolet-absorbing layerhaving the function of protecting the surface of the projection screen10, the function of preventing glaring, and the function of preventingstatic electrification; and (6) an ultraviolet-absorbing layer havingthe function of protecting the surface of the projection screen 10, thefunction of preventing reflection of light, and the function ofpreventing static electrification.

Thus, if other functions are imparted to the ultraviolet-absorbinglayer, a high-quality projection screen 10 having a simple structure canbe obtained at high efficiency.

Any ultraviolet light absorber may be used to form theultraviolet-absorbing layer as long as it has the property of absorbingultraviolet light. However, to ensure sufficient absorption ofultraviolet light, those ultraviolet light absorbers that are excellentin the absorption of ultraviolet light of 370 nm or shorter, and, fromthe viewpoint of image display performance, scarcely absorb visiblelight of 400 nm or more are preferred.

Specifically, salicylate, benzophenone, benzotriazole, benzoate,cyanoacrylate, or nickel-complex ultraviolet light absorbers may beused, for example. Of these, benzophenone, benzotriazole or salicylateultraviolet light absorbers are preferred.

Examples of benzophenone ultraviolet light absorbers include2,4-dihydroxybenzophenone, 2-hydroxy-4-acetoxybenzophenone,2-hydroxy-4-methoxybenzophenone, 2,2′-dihydroxy-4-methoxybenzo-phenone,2,2′-dihydroxy-4,4′-methoxybenzophenone,2-hydroxy-4-n-octoxybenzophenone, 2-hydroxy-4-dodecyloxybenzophenone,and 2-hydroxy-4-(2-hydroxy-3-methacryloxy)propoxybenzophenone.

Examples of benzotriazole ultraviolet light absorbers include2(2′-hydroxy-3′-tert-butyl-5′-methylphenyl)-5-chlorobenzotriazole,2(2′-hydroxy-5′-tert-butylphenyl)benzotriazole,2(2′-hydroxy-3′,5′-di-tert-amyl-phenyl)benzotriazole,2(2′-hydroxy-3′,5′-di-tert-butylphenyl)-5-chloro-benzotriazole, and2(2′-hydroxy-5′-tert-octylphenyl)benzotriazole.

Examples of salicylate ultraviolet light absorbers include phenylsalicylate, p-octylphenyl salicylate, and p-tert-butylphenyl salicylate.

Of the above-enumerated ultraviolet light absorbers,2-hydroxy-4-methoxybenzophenone,2,2′-dihydroxy-4,4′-methoxybenzophenone,2(2′-hydroxy-3′-tert-butyl-5′-methylphenyl)-5-chlorobenzotriazole,2(2′-hydroxy-5′-tert-butylphenyl)benzotriazole,2(2′-hydroxy-3′,5′-di-tert-amyl-phenyl)benzotriazole, and2(2′-hydroxy-3′,5,′-di-tert-butylphenyl)-5-chlorobenzotriazole arepreferred.

Any type of ultraviolet light absorbers can be used in this aspect ofthe invention. Moreover, it is possible to use a mixture of two or moredifferent types of ultraviolet light absorbers. The use of such amixture can produce a powerful shielding effect on ultraviolet light ina wider wave range.

The amount of the ultraviolet light absorber to be incorporated in theultraviolet-absorbing layer is preferably from 1 to 50 parts by weight,particularly from 5 to 40 parts by weight, for 100 parts by weight ofthe binder resin used for forming the ultraviolet-absorbing layer. Ifthe ultraviolet-absorbing layer contains an ultraviolet light absorberin an amount smaller than the above range, the layer cannot fully showthe function of absorbing ultraviolet light. On the other hand, If theultraviolet-absorbing layer contains an ultraviolet light absorber in anamount greater than the above range, the ultraviolet light absorbertends to bleed through the surface of the ultraviolet-absorbing layer.

Examples of the binder resin to which the ultraviolet light absorber isadded include phenoxy resins, vinyl resins, polyester resins,polystyrene resins, polyamide resins, polyurethane resins, and acrylicresins.

A method usually used to form such an ultraviolet-absorbing layer is asfollows: the above-described materials are dissolved or dispersed in aproper solvent to give an ultraviolet-absorbing-layer-forming coatingliquid, and this coating liquid is applied to the polarized-lightselective-reflection layer 11 formed on the substrate 12 and then driedand cured. Any ultraviolet-absorbing-layer-forming coating liquid can beused in the above method as long as the liquid contains theabove-described ultraviolet light absorber, binder resin, etc. dissolvedor dispersed in a proper solvent. Examples of solvents that can be usedin the ultraviolet-absorbing-layer-forming coating liquid includetoluene, ethyl acetate, butyl acetate, methyl ethyl ketone, methylisobutyl ketone, isopropyl alcohol and ethyl alcohol. To apply theultraviolet-absorbing-layer-forming coating liquid, any one of the knownmethods may be employed. Specifically, a roll, gravure, bar, slide, die,slit or dip coating method can be used. In the case where a plastic filmis used as the substrate 12, a film coating method using a so-calledroll-to-roll system may be used.

The thickness of the ultraviolet-absorbing layer is preferably between0.1 μm and 5 μm, particularly between 1 μm and 3 μm. When theultraviolet-absorbing layer has a thickness smaller than this range,there is a possibility that the function of absorbing ultraviolet lightcannot be satisfactorily obtained, and, if this layer also has otherfunctions such as the function of preventing reflection of light, thesefunctions may not be satisfactorily obtained. On the other hand, whenthe ultraviolet-absorbing layer has a thickness greater than theabove-described range, although these functions can be satisfactorilyobtained, the ultraviolet-absorbing layer may impede transmission ofimaging light projected from a projector, and the brightness thus dims.

(Antistatic Layer)

The antistatic layer is a member for preventing the projection screen 10from being electrostatically charged and is provided at least on eitherthe viewer's side or the backside surface of the polarized-lightselective reflection layer 11. The antistatic layer can have any surfaceresistivity as long as the layer can eliminate the static electricityproduced on the projection screen 10. It is, however, preferable thatthe surface resistivity of the antistatic layer be not more than 1×10¹¹Ω/□. As long as the antistatic layer has a surface resistivity in thisrange, the layer can satisfactorily prevent static electrification ofthe projection screen 10. If the antistatic layer is provided on theviewer's side surface of the polarized-light selective reflection layer11, the surface of the projection screen 10 does not produce staticelectricity that draws dust. On the other hand, if provided on the backsurface of the polarized-light selective reflection layer 11, theantistatic layer can prevent discharge of static electricity to a humanbody. Moreover, this antistatic layer can prevent deformation of theprojection screen 10 that is caused when the projection screen 10 isattracted to a wall or furniture by static electricity in the case wherethe projection screen 10 is in the form of a roll screen, for example.Of course, it is possible to provide the antistatic layer on both theviewer's side and the backside surfaces of the polarized-light selectivereflection layer 11. If the antistatic layer is provided on both sidesof the polarized-light selective reflection layer 11, the staticelectrification of the projection screen 10 is more satisfactorilyprevented. The antistatic layer is usually provided on the outermost,viewer's side or backside surface of the projection screen 10. Thisarrangement is used because, if the antistatic layer is so provided,those troubles that are caused by static electricity can be moreeffectively avoided.

In addition to the function of preventing the projection screen 10 frombeing electrostatically charged, the antistatic layer that is providedat least on the viewer's side or backside surface of the polarized-lightselective reflection layer 11 may have other functions. It is preferablethat these other functions include at least one of the followingfunctions: the function of preventing the surface of the projectionscreen 10 from being scratched, the function of preventing theprojection screen 10 from glaring, the function of preventing theprojection screen 10 from reflecting light, and the function ofabsorbing ultraviolet light that enters the projection screen 10. Inother words, in the case where provided on the viewer's side of thepolarized-light selective reflection layer 11 are a hard coat layer forpreventing the surface of the projection screen 10 from being scratchedand staining, an anti-glaring layer for preventing the projection screen10 from glaring, an anti-reflection layer for preventing the projectionscreen 10 from reflecting light, and an ultraviolet-absorbing layer forabsorbing ultraviolet light that enters the projection screen 10, if anantistatic agent or the like is incorporated in these layers, there canbe obtained: an antistatic layer having the function of protecting thesurface of the projection screen 10; an antistatic layer having thefunction of preventing glaring; an antistatic layer having the functionof preventing reflection of light; and an antistatic layer having thefunction of absorbing ultraviolet light.

The function of protecting the surface of projection screen 10 and thefunction of absorbing ultraviolet light may be imparted to such anantistatic layer together with either the function of preventing glaringor the function of preventing reflection of light. Examples ofantistatic layers having these functions are as follows: (1) anantistatic layer having the function of protecting the surface of theprojection screen 10 and the function of preventing glaring; (2) anantistatic layer having the function of protecting the surface of theprojection screen 10 and the function of preventing reflection of light;(3) an antistatic layer having the function of preventing glaring andthe function of absorbing ultraviolet light; (4) an antistatic layerhaving the function of preventing reflection of light and the functionof absorbing ultraviolet light; (5) an antistatic layer having thefunction of protecting the surface of the projection screen 10, thefunction of preventing glaring, and the function of absorbingultraviolet light; and (6) an antistatic layer having the function ofprotecting the surface of the projection screen 10, the function ofpreventing reflection of light, and the function of absorbingultraviolet light.

Thus, if other functions are imparted to the antistatic layer, there canbe obtained, at high efficiency, a high-quality projection screen 10having a simple structure.

To form such an antistatic layer, one of the following methods may beemployed: (1) a method in which a metal or metallic oxide havingantistatic properties is deposited or applied; (2) a method in which asilicate compound is applied; or (3) a method in which anantistatic-layer-forming coating liquid containing an antistatic agentis applied. In these methods (1) to (3), any one of the known methodsmay be employed to apply the various materials. Specifically, a roll,gravure, bar, slide, die, slit or dip coating method can be used. In thecase where a plastic film is used as the substrate 12, a film coatingmethod using a so-called roll-to-roll system may be used.

Specifically, any metal or metallic oxide can be used in method (1) aslong as the metal or metallic oxide has a surface resistivitysufficiently low that the resulting layer has antistatic properties.Examples of metals or metallic oxides useful herein include such metalsas tin, aluminum, silicon, copper, silver, gold and indium, and oxidesor double oxides of these metals. In the case where the antistatic layeris provided on the viewer's side of the polarized-light selectivereflection layer 11, it is preferable that this layer be transparent.From this point of view, it is preferable to deposit indium oxide orsilica.

From the same reason, the above-described method (2) using silicatecompounds, which are highly transparent, preferably is employed to formthe antistatic layer.

In the above-described method (3), any antistatic agent can be used aslong as it can make the surface resistivity of the resulting antistaticlayer low enough to prevent static electrification. Examples ofantistatic agents that can be used in this method include a variety ofsurface-active agent type antistatic agents such as: various cationicantistatic agents including quaternary ammonium salts, pyridinium salts,and those agents having cationic groups such as primary, secondary ortertiary amino groups; anionic antistatic agents having anionic groupssuch as sulfonic acid base, sulfuric ester base, phosphoric ester baseand phosphonic acid base; amphoteric antistatic agents such asamino-acid- or aminosulfuric-ester-containing agents; and nonionicantistatic agents such as aminoalcohol-, glycerin- orpolyethylene-glycol-containing agents.

The antistatic layer formed by method (3) includes a layer containing anantistatic agent as a main component, and a layer containing a properbinder resin to which an antistatic agent is added. In the latter case,any binder resin can be used as long as it is highly transparent anddoes not impair the antistatic properties of the antistatic agent. Ifsuch a function as the function of protecting the surface of theprojection screen 10, the function of preventing glaring, the functionof preventing reflection of light, or the function of absorbingultraviolet light is imparted to the antistatic layer, a resin materialor the like useful for forming a layer having such a function is usuallyused as the binder resin. Specifically, for example, to form anantistatic layer having the function of protecting the surface of theprojection screen 10, any of thermosetting resins, thermoplastic resins,ultraviolet-curing resins, electron-beam-curing resins and two-partresins may be used as the binder resin. To obtain an antistatic layerhaving the function of absorbing ultraviolet light, any of phenoxyresins, vinyl resins, polyester resins, polystyrene resins, polyamideresins, polyurethane resins, acrylic resins, etc. may be used as thebinder resin.

In the case where the antistatic layer is formed by adding an antistaticagent in a binder resin or the like, the content of the antistatic agentin the antistatic layer is preferably from 1 to 50% by weight,particularly from 5 to 20% by weight. When the content of the antistaticagent is lower than this range, the antistatic layer may not have afully lowered surface resistivity. On the other hand, when the contentof the antistatic agent is higher than the above-described range, theantistatic layer may have a poor appearance and a lowered heatdistortion temperature.

The antistatic layer can have any surface resistivity as long as it canprevent static electrification of the projection screen 10, However, thesurface resistivity of the antistatic layer is preferably 1×10¹¹ Ω/□ orless, particularly 1×10¹⁰ Ω/□ or less, more preferably 1×10⁹ Ω/□ orless. As long as the antistatic layer has a surface resistivity in theabove range, the layer can prevent generation of static electricity, sothat the projection screen 10 is prevented from being electrostaticallycharged and thus from being covered with dust. The above-describedsurface resistivity is the value obtained by moisture-conditioning asample in an atmosphere of 23° C. and 50% RH for 24 hours and measuringthe surface resistivity of this sample in accordance with ASTM D257,using a super megohmmeter.

The thickness of the antistatic layer is preferably from 0.5 to 5 μm,particularly from 1 to 2 μm. If the antistatic layer has a thicknesssmaller than this range, the function of preventing staticelectrification of the projection screen 10, and other functions such asthe function of preventing glaring, if imparted, may not besatisfactorily obtained. On the other hand, if the antistatic layer hasa thickness greater than the above-described range, although thesefunctions can be satisfactorily obtained, the antistatic layer mayimpede transmission of imaging light projected from a projector, and thebrightness thus dims.

(Combination of Functional Layers)

Some of or all of the above-described functions, that is, the functionof protecting the surface of the projection screen 10, the function ofpreventing glaring, the function of preventing reflection of light, thefunction of absorbing ultraviolet light, and the function of preventingstatic electrification can be imparted to one layer, as mentioned above.Alternatively, these functions may be imparted to separate layers, andthese separate layers may be layered on the polarized-light selectivereflection layer 11. Specifically, for example, the anti-reflectionlayer, the anti-glaring layer, the ultraviolet-absorbing layer and thehard coat layer may be layered on the polarized-light selectivereflection layer 11 in the order mentioned. Although it is necessarythat the hard coat layer be provided on the outermost, viewer's sidesurface of the projection screen 10, the other layers may be layered inany order. The ultraviolet-absorbing layer is not necessarily a singlelayer, and a plurality of ultraviolet-absorbing layers may be provided.For example, by incorporating an ultraviolet light absorber in any twoor more of the anti-reflection layer, the anti-glaring layer and thehard coat layer, it is possible to provide a plurality ofultraviolet-absorbing layers. Although the antistatic layer is providedon the viewer's side or backside surface of the polarized-lightselective reflection layer 11, it is preferable not to provide thislayer on the viewer's side of the anti-reflection layer, for example.This arrangement is preferred because if the antistatic layer isprovided on the viewer's side of the anti-reflection layer, the functionof the anti-reflection layer, that is, the function of preventingreflection of extraneous light, may be impaired.

Projection System

The aforementioned projection screen 10 can be incorporated into aprojection system 20 having a projector 21, as shown in FIG. 13.

As shown in FIG. 13, the projection system 20 includes the projectionscreen 10 and the projector 21 for projecting imaging light on theprojection screen 10.

Of these components, the projector 21 may be of any type, and it ispossible to use a CRT projector, a liquid crystal projector, a DLP(digital light processing) projector, or the like. It is, however,preferable that the imaging light to be projected on the projectionscreen 10 from the projector 21 contains chiefly a polarized lightcomponent (e.g., right-handed circularly polarized light) that isidentical with the polarized light component that the projection screen10 selectively reflects.

A liquid crystal projector useful as the projector 21 usually emitssubstantially linearly polarized light because of the operatingprinciple of this projector. In this case, by letting the imaging lightemerge from the projector 21 through a retardation layer 22 or the like,it is possible to convert the linearly polarized light into circularlypolarized light without loss of the amount of light.

A quarter wave plate is preferable as the retardation layer 22.Specifically, an ideal retardation layer is one capable of causing aphase shift of 137.5 nm for light with a wavelength of 550 nm at whichvisibility is maximized. Further, a wide-wave-range quarter wave plateis more preferable because this wave plate is applicable to light in allof the red (R), green (G) and blue (B) color wave ranges. It is alsopossible to use a single retardation layer made by controlling thebirefringence of a material for this layer, or a retardation layer usinga quarter wave plate in combination with a half wave plate.

The retardation layer 22 may be externally attached to the exit apertureof the projector 21, as shown in FIG. 13, or incorporated into theinside of the projector 21.

In the case where a CRT or DLP projector is used as the projector 21,because the projector emits unpolarized light, it is necessary toprovide a circular polarizer composed of a linear polarizer and aretardation layer in order to convert the unpolarized light intocircularly polarized light. if a circular polarizer is provided,although the amount of light emitted from the projector 21 itself isdecreased to half, stray light or the like is not produced from apolarized light component (e.g., left-handed circularly polarized light)that is different from the polarized light that the polarized-lightselective reflection layer 11 in the projection screen 10 selectivelyreflects, so that image contrast is enhanced.

A projector 21 usually attains color display utilizing light in the waveranges for red (R), green (G) and blue (B) colors, the three primarycolors. For example, assuming that light enters the projection screen 10vertically to it, the projector 21 projects light in wave ranges whosecenters are between 430 nm and 460 nm, between 540 nm and 570 nm, andbetween 580 nm and 620 nm. For this reason, it is preferable to make theprojection screen 10 selectively reflect only light in wave rangesidentical to the wave ranges in which the imaging light projected fromthe projector 21 fall. If the projection screen 10 is so made, of theenvironmental light such as sunlight and light from lighting fixtures,those lights in the visible region not in the above-described waveranges are not reflected at the projection screen 10, so that imagecontrast is enhanced.

The projection system 20 usually includes an illuminant 23 that is fixedto an illuminant-fixing position 25 such as the ceiling of a room, andthis illuminant 23 illuminates a space in which the projection screen 10is placed.

As shown in FIG. 13, in the case where the illuminant 23 is sopositioned that the light emitted from the illuminant 23 illuminatesdirectly the projection screen 10, it is preferable that the light 34emitted from the illuminant 23 toward the projection screen 10 be madeto contain mainly a polarized light component (e.g., left-handedcircularly polarized light) that is different from the polarized lightcomponent that the projection screen 10 selectively reflects. By somaking the light 34, it is possible effectively to prevent thepolarized-light selective reflection layer in the projection screen 10from reflecting the light 34, thereby enhancing image contrast.

The state of polarization of the light 34 that is emitted from theilluminant 23 can be controlled by providing, in the vicinity of theilluminant 23, a polarizer film 24 capable of transmitting left-handedcircularly polarized light. It is herein possible to use, as thepolarizer film 24, an absorption circular polarizer or a polarized-lightseparator (reflection circular polarizer). Examples of polarized-light.separators useful herein include: circularly-polarized-light separatorsusing cholesteric liquid crystal layers; and linearly-polarized-lightseparators containing, on the exit side, retardation layers forconverting linearly polarized light into circularly polarized light.Such polarized-light separators are preferred because they cause only asmall loss of the amount of light as compared to absorption circularpolarizers.

In the projection system 20 shown in FIG. 13, the light 34 emitted fromthe illuminant 23 illuminates directly the projection screen 10, Thepresent invention is not limited to this arrangement and also includesthe case where the illuminant 23 is, as shown in FIG. 14, mounted on anilluminant-mounting position 26 other than the ceiling so that the light35 emitted from the illuminant 23 illuminates, as light 35′, indirectlythe projection screen 10 via a reflector 27 such as the ceiling. In thiscase, the state of polarization of the circularly polarized light isreversed when the light is reflected at the reflector 27. It is,therefore, preferable that the light 35 emitted from the illuminant 23toward the reflector 27 be made to contain mainly a polarized lightcomponent (e.g., right-handed circularly polarized light) that isidentical to the polarized light component that the projection screen 10selectively reflects, by providing a polarizer film 24′ or the like thattransmits right-handed circularly polarized light, as in the case shownin FIG. 13. The polarizer film 24′ may be the same as theabove-described polarizer film 24. If such a polarizer film is used, thelight 35′ whose state of polarization has been reversed by the reflector27 is to contain mainly a polarized light component (e.g., left-handedcircularly polarized light) that is different from the polarized lightcomponent that the projection screen 10 selectively reflects. For thisreason, the light 35′ is not reflected from the polarized-lightselective reflection layer 11 in the projection screen 10, and imagecontrast is thus enhanced.

EXAMPLES

The present invention will now be explained more specifically byreferring to the following examples. However, these examples are notintended to limit or restrict the scope of the invention in any way.

Example 1

A first cholesteric liquid crystal solution having a selectivereflection wave range centered at 440 nm was prepared by dissolving, incyclohexanone, a monomer-containing liquid crystal consisting of a maincomponent (94.7% by weight), an ultraviolet-curing nematic liquidcrystal, and a polymerizable chiral agent (5.3% by weight). A liquidcrystal containing a compound represented by the above chemical formula(2-xi) was used as the nematic liquid crystal. A compound represented bythe above chemical formula (5) was used as the polymerizable chiralagent. To the first cholesteric liquid crystal solution, 5% by weight of“Irg 369” available from Ciba Specialty Chemicals K.K., Japan was addedas a photopolymerization initiator.

By a bar coating method, the above-prepared first cholesteric liquidcrystal solution was applied to a substrate (“LUMIRROR/AC-X”manufactured by Panac Co., Ltd., Japan) that was a black-colored PETfilm with a surface area of 200 mm×200 mm, having thereon an adherentlayer.

This resulting layer was subjected to an aligning treatment (dryingtreatment) by heating in an oven at 80° C. for 90 seconds, whereby acholesteric liquid crystal layer containing no solvent was obtained.

Thereafter, 50 mW/cm² of ultraviolet light with a wavelength of 365 nmwas applied to this cholesteric liquid crystal layer in an atmosphere ofnitrogen for one minute to cure the cholesteric liquid crystal layer.Thus, a first partial selective reflection layer having a selectivereflection wave range centered at 440 nm was obtained.

Similarly, a second cholesteric liquid crystal solution was applieddirectly to the first partial selective reflection layer and thensubjected to alignment treatment (drying treatment) and curingtreatment. Thus, a second partial selective reflection layer having aselective reflection wave range centered at 550 nm was obtained. Theprocedure used for preparing the second liquid crystal solution was thesame as the procedure used for preparing the first liquid crystalsolution, provided that the nematic liquid crystal and the chiral agentwere mixed in such a proportion that the resulting layer had a selectivereflection wave range centered at 550 nm.

Similarly, a third cholesteric liquid crystal solution was applieddirectly to the second partial selective reflection layer and thensubjected to alignment treatment (drying treatment) and curingtreatment. Thus, a third partial selective reflection layer having aselective reflection wave range centered at 600 nm was obtained. Theprocedure used for preparing the third liquid crystal solution was thesame as the procedure used for preparing the first liquid crystalsolution, provided that the nematic liquid crystal and the chiral agentwere mixed in such a proportion that the resulting layer had a selectivereflection wave range centered at 600 nm.

Thus, there was obtained a projection screen having a polarized-lightselective reflection layer composed of the first partial selectivereflection layer capable of selectively reflecting light in the blue (B)color wave range (light in a selective reflection wave range centered at440 nm), the second partial selective reflection layer capable ofselectively reflecting light in the green (G) color wave range (light ina selective reflection wave range centered at 550 nm), and the thirdpartial selective reflection layer capable of selectively reflectinglight in the red (R) color wave range (light in a selective reflectionwave range centered at 600 nm), layered in the order mentionedsuccessively from the substrate side. The first partial selectivereflection layer was 3 μm thick, the second partial selective reflectionlayer was 4 μm thick, and the third partial selective reflection layerwas 5 μm thick. These partial selective reflection layers constitutingthe polarized-light selective reflection layer in the projection screenhad cholesteric liquid crystalline structures that were not in the stateof planar orientation.

From a photomicrograph of the sectional structure taken by atransmission electron photomicroscope (a sectional TEM photo), 10edge-shaped-structure parts were observed in the cross section with abase length of 100 μm and a height of 1 μm. Moreover, both thosehelical-structure parts in which the helical axes were tilted clockwiseat an angle of 10° with the normal and those helical-structure parts inwhich the helical axes were tilted counterclockwise at an angle of 10°with the normal were confirmed to be present in this cross section. Usedfor these observations was a specimen obtained by slicing thepolarized-light selective reflection layer embedded in an epoxy resinand subjecting this slice to double staining; a “JEM-200CX” transmissionelectron photomicroscope manufactured by JEOL, Ltd., Japan was alsoused.

The polarized-light selective reflection layer in the projection screenobtained in the above-described manner was found to have an angle ofdiffusion of ±30°. The “angle of diffusion” herein refers to themeasured angle at which the reflectivity is equal to ⅓ of the maximumreflectivity (excluding reflectivity originating from interfacialreflection), obtained by projecting light on a projection screen at anangle of 30° with respect to the normal of the screen and measuring theangle at which this light is back-scattered, where the measured angle atwhich the reflectivity reaches a maximum is 0°.

Example 2

“ADEKA Optomer KRX-559-7” manufactured by ASAHI DENKA KOGYO K.K., Japanwas prepared as a material for forming a hard coat layer, and wasapplied, by a bar coating method, directly to the polarized-lightselective reflection layer in the projection screen obtained inExample 1. This coating film was dried at 80° C. for 5 minutes and wasthen cured by the application of 750 mJ/cm² of ultraviolet light,thereby forming a hard coat layer with a thickness of 5 μm. There wasthus obtained a projection screen containing the hard coat layerlaminated to the polarized-light selective reflection layer. The surfacehardness of the hard coat layer in the projection screen obtained inthis manner was measured. As a result, the pencil hardness of the hardcoat layer determined in accordance with JIS K5400 was found to be 2H ormore.

Example 3

“ZR-100 (trade name)” manufactured by Sumitomo Osaka Cement Co., Ltd.,Japan was prepared as a material for forming an ultraviolet-absorbinglayer, and was applied, by a spin coating method, directly to thepolarized-light selective reflection layer in the projection screenobtained in Example 1. This coating film was dried at 80° C. for 1minute, thereby obtaining an ultraviolet-absorbing layer whose thicknesswas approximately 2 μm. Thus, there was obtained a projection screen inwhich the ultraviolet-absorbing layer was laminated to thepolarized-light selective reflection layer.

Example 4

A xylene solution (solid matter 30%) containing polyacrylic ester as abinder and tin oxide in an amount of 30% of the binder was prepared as amaterial for forming an antistatic layer, and was applied, by a barcoating method, directly to the polarized-light selective reflectionlayer in the projection screen obtained in Example 1. This coating filmwas dried at 80° C. and was then cured by the application of 100 mJ/cm²of ultraviolet light, thereby obtaining an antistatic layer with athickness of 1 μm. Thus, there was obtained a projection screencontaining the antistatic layer laminated to the polarized-lightselective reflection layer. The surface resistivity of the antistaticlayer in the projection screen obtained in this manner was 1×10¹¹ Ω/□.

Comparative Example 1

A projection screen was produced in the same manner as in Example 1,provided that a stretched, black-colored PET film (“LUMIRROR”manufactured by Panac Co., Ltd., Japan) was used as the substrate. Thesurface of the polarized-light selective reflection layer in theprojection screen obtained in this manner was found to be in the stateof planar orientation. The partial selective reflection layersconstituting the polarized-light selective reflection layer were foundto have cholesteric liquid crystalline structures that were also in thestate of planar orientation.

Comparative Example 2

A commercially available projection screen manufactured by OS Co., Ltd.,Japan, composed of cloth and a beads-containing scattering layer formedon the cloth surface, was prepared.

(Results of Evaluation)

Imaging light emitted from a projector was projected on each one of theprojection screens of Example 1 and Comparative Examples 1 and 2, andthe contrast values were determined. In this measurement, a liquidcrystal projector (“ELP-52” manufactured by Seiko Epson Corporation,Japan) was used as the projector.

In order to convert the imaging light emitted from the projector intocircularly polarized light, a circular polarizer was placed on the exitaperture of the projector. A fluorescent lamp (emitting unpolarizedlight) fixed to the ceiling was used to illuminate the room in which theprojector and each projection screen were placed, where the projectionscreen and the fluorescent lamp were arranged so that the light from thefluorescent light directly entered the projection screen at an angle ofapproximately 50°. The illumination intensity on the projection screenright under the fluorescent lamp, measured by an illuminometer (adigital illuminometer “510-02” manufactured by Yokogawa M&C Corporation,Japan), was 200 1×.

The projection screen was set vertically to the floor. The projector wasplaced at such a point that the horizontal distance (in parallel withthe floor) between the projector and the projection screen wasapproximately 2.5 m.

Imaging light (a still image composed of white and black areas) wasprojected on the projection screen from the projector, and the imagecontrast was determined. Specifically, the luminance of the white areaand that of the black area in the center of the projection screen weremeasured by a luminance meter “BM-8” manufactured by Topcon Corp.,Japan, and the ratio between these two luminances was obtained as theimage contrast [contrast=(luminance of white area)÷(luminance of blackarea)].

The contrast values of the images projected on the projection screens ofExample 1 and Comparative Examples 1 and 2 are shown in Table 1.

Further, these projection screens were observed visually. On theprojection screen of Comparative Example 1, because specular reflectionof the projected light occurred, it was difficult to recognize thereflected light as an image and it was impossible to measure theluminances. The images displayed on the projection screens of Example 1and Comparative Example 2 were well recognizable; however, the contrastof the image on the projection screen of Example 1 was approximately 8times higher than the contrast of the image on the projection screen ofComparative Example 2.

TABLE 1 Sample Comparative Comparative Example 1 Ex. 1 Ex. 2 Contrast 30— 4

On the other hand, the projection screens of Examples 1 and 2 weresubjected to abrasion tests. Compared with the projection screen ofExample 1, the projection screen of Example 2 was scarcely scratched.

The projection screens of Examples 1 and 3 were subjected to 200-hourweatherability tests using a weatherometer (“SLLM-U” manufactured bySuga Test Instruments Co., Ltd., Japan). The image projected on eachprojection screen was observed before and after the test. As a result,the image projected on the projection screen of Example 3, observedafter the test, was found to be the same in color tone as the imageobserved before the test, and in this respect, this projection screenwas superior to the projection screen of Example 1.

The projection screens of Examples 1 and 4 were left to stand for 24hours. Compared with the projection screen of Example 1, the projectionscreen of Example 4 was scarcely covered with dust.

1. A projection screen that displays an image by reflecting projectedimaging light, the screen comprising: a polarized-light selectivereflection layer having a cholesteric liquid crystalline structure andadapted to selectively reflect a specific polarized light component;wherein the polarized-light selective reflection layer selectivelyreflects the light component while diffusing the light component as aresult of structural non-uniformity in the cholesteric liquidcrystalline structure.
 2. The projection screen according to claim 1,wherein the cholesteric liquid crystalline structure of thepolarized-light selective reflection layer comprises a plurality ofhelical-structure parts that have different directions of helical axes.3. The projection screen according to claim 1, wherein thepolarized-light selective reflection layer selectively reflects light ina specific wave range that covers only a part of the visible region. 4.The projection screen according to claim 3, wherein the polarized-lightselective reflection layer has, for light in a wave range that coversonly a part of the visible region, a reflectivity not less than half themaximum reflectivity of this layer.
 5. The projection screen accordingto claim 3, wherein assuming that light enters the polarized-lightselective reflection layer vertically to it, the polarized-lightselective reflection layer selectively reflects light in wave rangeswhose centers are between 430 nm and 460 nm, between 540 nm and 570 nm,and between 580 nm and 620 nm.
 6. The projection screen according toclaim 1, further comprising, on an outermost, viewer's side surface ofthe polarized-light selective reflection layer, a hard coat layer forpreventing the surface of the projection screen from being scratched. 7.The projection screen according to claim 6, wherein the hard coat layerhas a surface hardness of 2H or more when expressed by the pencilhardness.
 8. The projection screen according to claim 1, furthercomprising, on a viewer's side of the polarized-light selectivereflection layer, an anti-glaring layer for preventing the projectionscreen from glaring.
 9. The projection screen according to claim 1,further comprising, on a viewer's side of the polarized-light selectivereflection layer, an anti-reflection layer for preventing the projectionscreen from reflecting extraneous light.
 10. A projection systemcomprising: a projection screen that displays an image by reflectingprojected imaging light, the screen comprising: a polarized-lightselective reflection layer having a cholesteric liquid crystallinestructure and adapted to selectively reflect a specific polarized lightcomponent, wherein the polarized-light selective reflection layerselectively reflects the light component while diffusing the lightcomponent as a result of structural non-uniformity in the cholestericliquid crystalline structure; and a projector that projects imaginglight on the projection screen.
 11. The projection system according toclaim 10, wherein the projection screen selectively reflects light in awave range that is identical to a wave range in which the imaging lightprojected from the projector falls.
 12. The projection system accordingto claim 10, wherein the imaging light to be projected on the projectionscreen from the projector contains mainly a polarized light componentthat is identical to a polarized light component that the projectionscreen selectively reflects.
 13. The projection system according toclaim 10, further comprising an illuminant for illuminating a space inwhich the projection screen is placed, the illuminant being sopositioned that light emitted from the illuminant directly illuminatesthe projection screen, wherein the light emitted from the illuminanttoward the projection screen contains mainly a polarized light componentthat is different from a polarized light component that the projectionscreen selectively reflects.